diff --git a/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php b/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php
index bf5e0aae270..66e6a2ec038 100644
--- a/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryInterface.php
@@ -33,6 +33,8 @@ interface DiscoveryInterface {
    * @return mixed[]
    *   An array of plugin definitions (empty array if no definitions were
    *   found). Keys are plugin IDs.
+   *
+   * @see \Drupal\Core\Plugin\FilteredPluginManagerInterface::getFilteredDefinitions()
    */
   public function getDefinitions();
 
diff --git a/core/lib/Drupal/Core/Block/BlockManager.php b/core/lib/Drupal/Core/Block/BlockManager.php
index 30b52b61256..467e63565ef 100644
--- a/core/lib/Drupal/Core/Block/BlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockManager.php
@@ -6,8 +6,8 @@ use Drupal\Component\Plugin\FallbackPluginManagerInterface;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Plugin\CategorizingPluginManagerTrait;
-use Drupal\Core\Plugin\Context\ContextAwarePluginManagerTrait;
 use Drupal\Core\Plugin\DefaultPluginManager;
+use Drupal\Core\Plugin\FilteredPluginManagerTrait;
 
 /**
  * Manages discovery and instantiation of block plugins.
@@ -21,7 +21,7 @@ class BlockManager extends DefaultPluginManager implements BlockManagerInterface
   use CategorizingPluginManagerTrait {
     getSortedDefinitions as traitGetSortedDefinitions;
   }
-  use ContextAwarePluginManagerTrait;
+  use FilteredPluginManagerTrait;
 
   /**
    * Constructs a new \Drupal\Core\Block\BlockManager object.
@@ -37,10 +37,17 @@ class BlockManager extends DefaultPluginManager implements BlockManagerInterface
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
     parent::__construct('Plugin/Block', $namespaces, $module_handler, 'Drupal\Core\Block\BlockPluginInterface', 'Drupal\Core\Block\Annotation\Block');
 
-    $this->alterInfo('block');
+    $this->alterInfo($this->getType());
     $this->setCacheBackend($cache_backend, 'block_plugins');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function getType() {
+    return 'block';
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/lib/Drupal/Core/Block/BlockManagerInterface.php b/core/lib/Drupal/Core/Block/BlockManagerInterface.php
index 3455f23b638..7b5d5c6332b 100644
--- a/core/lib/Drupal/Core/Block/BlockManagerInterface.php
+++ b/core/lib/Drupal/Core/Block/BlockManagerInterface.php
@@ -4,10 +4,11 @@ namespace Drupal\Core\Block;
 
 use Drupal\Component\Plugin\CategorizingPluginManagerInterface;
 use Drupal\Core\Plugin\Context\ContextAwarePluginManagerInterface;
+use Drupal\Core\Plugin\FilteredPluginManagerInterface;
 
 /**
  * Provides an interface for the discovery and instantiation of block plugins.
  */
-interface BlockManagerInterface extends ContextAwarePluginManagerInterface, CategorizingPluginManagerInterface {
+interface BlockManagerInterface extends ContextAwarePluginManagerInterface, CategorizingPluginManagerInterface, FilteredPluginManagerInterface {
 
 }
diff --git a/core/lib/Drupal/Core/Condition/ConditionManager.php b/core/lib/Drupal/Core/Condition/ConditionManager.php
index 2c6b61b6ae8..89fe216a441 100644
--- a/core/lib/Drupal/Core/Condition/ConditionManager.php
+++ b/core/lib/Drupal/Core/Condition/ConditionManager.php
@@ -8,8 +8,9 @@ use Drupal\Core\Executable\ExecutableManagerInterface;
 use Drupal\Core\Executable\ExecutableInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Plugin\CategorizingPluginManagerTrait;
-use Drupal\Core\Plugin\Context\ContextAwarePluginManagerTrait;
 use Drupal\Core\Plugin\DefaultPluginManager;
+use Drupal\Core\Plugin\FilteredPluginManagerInterface;
+use Drupal\Core\Plugin\FilteredPluginManagerTrait;
 
 /**
  * A plugin manager for condition plugins.
@@ -20,10 +21,10 @@ use Drupal\Core\Plugin\DefaultPluginManager;
  *
  * @ingroup plugin_api
  */
-class ConditionManager extends DefaultPluginManager implements ExecutableManagerInterface, CategorizingPluginManagerInterface {
+class ConditionManager extends DefaultPluginManager implements ExecutableManagerInterface, CategorizingPluginManagerInterface, FilteredPluginManagerInterface {
 
   use CategorizingPluginManagerTrait;
-  use ContextAwarePluginManagerTrait;
+  use FilteredPluginManagerTrait;
 
   /**
    * Constructs a ConditionManager object.
@@ -43,6 +44,13 @@ class ConditionManager extends DefaultPluginManager implements ExecutableManager
     parent::__construct('Plugin/Condition', $namespaces, $module_handler, 'Drupal\Core\Condition\ConditionInterface', 'Drupal\Core\Condition\Annotation\Condition');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function getType() {
+    return 'condition';
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/lib/Drupal/Core/Layout/LayoutPluginManager.php b/core/lib/Drupal/Core/Layout/LayoutPluginManager.php
index 179d1ede9ab..a5a46529377 100644
--- a/core/lib/Drupal/Core/Layout/LayoutPluginManager.php
+++ b/core/lib/Drupal/Core/Layout/LayoutPluginManager.php
@@ -12,12 +12,15 @@ use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
 use Drupal\Core\Plugin\Discovery\ContainerDerivativeDiscoveryDecorator;
 use Drupal\Core\Plugin\Discovery\YamlDiscoveryDecorator;
 use Drupal\Core\Layout\Annotation\Layout;
+use Drupal\Core\Plugin\FilteredPluginManagerTrait;
 
 /**
  * Provides a plugin manager for layouts.
  */
 class LayoutPluginManager extends DefaultPluginManager implements LayoutPluginManagerInterface {
 
+  use FilteredPluginManagerTrait;
+
   /**
    * The theme handler.
    *
@@ -42,8 +45,16 @@ class LayoutPluginManager extends DefaultPluginManager implements LayoutPluginMa
     parent::__construct('Plugin/Layout', $namespaces, $module_handler, LayoutInterface::class, Layout::class);
     $this->themeHandler = $theme_handler;
 
-    $this->setCacheBackend($cache_backend, 'layout');
-    $this->alterInfo('layout');
+    $type = $this->getType();
+    $this->setCacheBackend($cache_backend, $type);
+    $this->alterInfo($type);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getType() {
+    return 'layout';
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Layout/LayoutPluginManagerInterface.php b/core/lib/Drupal/Core/Layout/LayoutPluginManagerInterface.php
index c0e606d41ff..340a80d39ba 100644
--- a/core/lib/Drupal/Core/Layout/LayoutPluginManagerInterface.php
+++ b/core/lib/Drupal/Core/Layout/LayoutPluginManagerInterface.php
@@ -3,11 +3,12 @@
 namespace Drupal\Core\Layout;
 
 use Drupal\Component\Plugin\CategorizingPluginManagerInterface;
+use Drupal\Core\Plugin\FilteredPluginManagerInterface;
 
 /**
  * Provides the interface for a plugin manager of layouts.
  */
-interface LayoutPluginManagerInterface extends CategorizingPluginManagerInterface {
+interface LayoutPluginManagerInterface extends CategorizingPluginManagerInterface, FilteredPluginManagerInterface {
 
   /**
    * Gets theme implementations for layouts.
diff --git a/core/lib/Drupal/Core/Plugin/Context/ContextAwarePluginManagerInterface.php b/core/lib/Drupal/Core/Plugin/Context/ContextAwarePluginManagerInterface.php
index 5c29411f32b..60013491cd2 100644
--- a/core/lib/Drupal/Core/Plugin/Context/ContextAwarePluginManagerInterface.php
+++ b/core/lib/Drupal/Core/Plugin/Context/ContextAwarePluginManagerInterface.php
@@ -20,6 +20,8 @@ interface ContextAwarePluginManagerInterface extends PluginManagerInterface {
    *
    * @return array
    *   An array of plugin definitions.
+   *
+   * @see \Drupal\Core\Plugin\FilteredPluginManagerInterface::getFilteredDefinitions()
    */
   public function getDefinitionsForContexts(array $contexts = []);
 
diff --git a/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
index 56dcdb2a1c1..d276a553aa1 100644
--- a/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
+++ b/core/lib/Drupal/Core/Plugin/Context/ContextHandler.php
@@ -17,7 +17,9 @@ class ContextHandler implements ContextHandlerInterface {
   public function filterPluginDefinitionsByContexts(array $contexts, array $definitions) {
     return array_filter($definitions, function ($plugin_definition) use ($contexts) {
       // If this plugin doesn't need any context, it is available to use.
-      if (!isset($plugin_definition['context'])) {
+      // @todo Support object-based plugin definitions in
+      //   https://www.drupal.org/project/drupal/issues/2961822.
+      if (!is_array($plugin_definition) || !isset($plugin_definition['context'])) {
         return TRUE;
       }
 
diff --git a/core/lib/Drupal/Core/Plugin/FilteredPluginManagerInterface.php b/core/lib/Drupal/Core/Plugin/FilteredPluginManagerInterface.php
new file mode 100644
index 00000000000..0404a66f8ee
--- /dev/null
+++ b/core/lib/Drupal/Core/Plugin/FilteredPluginManagerInterface.php
@@ -0,0 +1,36 @@
+<?php
+
+namespace Drupal\Core\Plugin;
+
+use Drupal\Component\Plugin\PluginManagerInterface;
+
+/**
+ * Provides an interface for plugin managers that allow filtering definitions.
+ */
+interface FilteredPluginManagerInterface extends PluginManagerInterface {
+
+  /**
+   * Gets the plugin definitions for a given type and consumer and filters them.
+   *
+   * This allows modules and themes to alter plugin definitions at runtime,
+   * which is useful for tasks like hiding specific plugins from a particular
+   * user interface.
+   *
+   * @param string $consumer
+   *   A string identifying the consumer of these plugin definitions.
+   * @param \Drupal\Component\Plugin\Context\ContextInterface[]|null $contexts
+   *   (optional) Either an array of contexts to use for filtering, or NULL to
+   *   not filter by contexts.
+   * @param mixed[] $extra
+   *   (optional) An associative array containing additional information
+   *   provided by the code requesting the filtered definitions.
+   *
+   * @return \Drupal\Component\Plugin\Definition\PluginDefinitionInterface[]|array[]
+   *   An array of plugin definitions that are filtered.
+   *
+   * @see hook_plugin_filter_TYPE_alter()
+   * @see hook_plugin_filter_TYPE__CONSUMER_alter()
+   */
+  public function getFilteredDefinitions($consumer, $contexts = NULL, array $extra = []);
+
+}
diff --git a/core/lib/Drupal/Core/Plugin/FilteredPluginManagerTrait.php b/core/lib/Drupal/Core/Plugin/FilteredPluginManagerTrait.php
new file mode 100644
index 00000000000..e21c367cc03
--- /dev/null
+++ b/core/lib/Drupal/Core/Plugin/FilteredPluginManagerTrait.php
@@ -0,0 +1,75 @@
+<?php
+
+namespace Drupal\Core\Plugin;
+
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\Context\ContextAwarePluginManagerTrait;
+use Drupal\Core\Theme\ThemeManagerInterface;
+
+/**
+ * Provides a trait for plugin managers that allow filtering plugin definitions.
+ */
+trait FilteredPluginManagerTrait {
+
+  use ContextAwarePluginManagerTrait;
+
+  /**
+   * Implements \Drupal\Core\Plugin\FilteredPluginManagerInterface::getFilteredDefinitions().
+   */
+  public function getFilteredDefinitions($consumer, $contexts = NULL, array $extra = []) {
+    if (!is_null($contexts)) {
+      $definitions = $this->getDefinitionsForContexts($contexts);
+    }
+    else {
+      $definitions = $this->getDefinitions();
+    }
+
+    $type = $this->getType();
+    $hooks = [];
+    $hooks[] = "plugin_filter_{$type}";
+    $hooks[] = "plugin_filter_{$type}__{$consumer}";
+    $this->moduleHandler()->alter($hooks, $definitions, $extra, $consumer);
+    $this->themeManager()->alter($hooks, $definitions, $extra, $consumer);
+    return $definitions;
+  }
+
+  /**
+   * A string identifying the plugin type.
+   *
+   * This string should be unique and generally will correspond to the string
+   * used by the discovery, e.g. the annotation class or the YAML file name.
+   *
+   * @return string
+   *   A string identifying the plugin type.
+   */
+  abstract protected function getType();
+
+  /**
+   * Wraps the module handler.
+   *
+   * @return \Drupal\Core\Extension\ModuleHandlerInterface
+   *   The module handler.
+   */
+  protected function moduleHandler() {
+    if (property_exists($this, 'moduleHandler') && $this->moduleHandler instanceof ModuleHandlerInterface) {
+      return $this->moduleHandler;
+    }
+
+    return \Drupal::service('module_handler');
+  }
+
+  /**
+   * Wraps the theme manager.
+   *
+   * @return \Drupal\Core\Theme\ThemeManagerInterface
+   *   The theme manager.
+   */
+  protected function themeManager() {
+    if (property_exists($this, 'themeManager') && $this->themeManager instanceof ThemeManagerInterface) {
+      return $this->themeManager;
+    }
+
+    return \Drupal::service('theme.manager');
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Plugin/plugin.api.php b/core/lib/Drupal/Core/Plugin/plugin.api.php
new file mode 100644
index 00000000000..5c85c1162ec
--- /dev/null
+++ b/core/lib/Drupal/Core/Plugin/plugin.api.php
@@ -0,0 +1,68 @@
+<?php
+
+/**
+ * @file
+ * Hooks provided by the Plugin system.
+ */
+
+/**
+ * @addtogroup hooks
+ * @{
+ */
+
+/**
+ * Alter the filtering of plugin definitions for a specific plugin type.
+ *
+ * TYPE (e.g. "block", "layout") limits hook scope to a plugin type.
+ * For example, HOOK_plugin_filter_block_alter() would be invoked
+ * by a hook listener which specifies the 'block' plugin list,
+ * e.g., BlockLibraryController or ChooseBlockController.
+ *
+ * @param \Drupal\Component\Plugin\Definition\PluginDefinitionInterface[]|array[] $definitions
+ *   The array of plugin definitions.
+ * @param mixed[] $extra
+ *   An associative array containing additional information provided by the code
+ *   requesting the filtered definitions.
+ * @param string $consumer
+ *   A string identifying the consumer of these plugin definitions.
+ */
+function hook_plugin_filter_TYPE_alter(array &$definitions, array $extra, $consumer) {
+  // Remove the "Help" block from the Block UI list.
+  if ($consumer == 'block_ui') {
+    unset($definitions['help_block']);
+  }
+
+  // If the theme is specified, remove the branding block from the Bartik theme.
+  if (isset($extra['theme']) && $extra['theme'] === 'bartik') {
+    unset($definitions['system_branding_block']);
+  }
+
+  // Remove the "Main page content" block from everywhere.
+  unset($definitions['system_main_block']);
+}
+
+/**
+ * Alter the filtering of plugin definitions for a specific type and consumer.
+ *
+ * TYPE (e.g. "block", "layout") limits hook scope to a plugin type.
+ * CONSUMER (e.g., "block_ui", "layout_builder") limits hook scope to one or
+ * more listeners, typically provided the same module. For example,
+ * HOOK_plugin_filter_layout__layout_builder_alter() would affect
+ * Layout Builder's listeners for the 'layout' plugin type (see
+ * ChooseSectionController), while HOOK_plugin_filter_block__block_ui_alter()
+ * would affect the Block UI's listeners for the 'block' plugin type.
+ *
+ * @param \Drupal\Component\Plugin\Definition\PluginDefinitionInterface[]|array[] $definitions
+ *   The array of plugin definitions.
+ * @param mixed[] $extra
+ *   An associative array containing additional information provided by the code
+ *   requesting the filtered definitions.
+ */
+function hook_plugin_filter_TYPE__CONSUMER_alter(array &$definitions, array $extra) {
+  // Explicitly remove the "Help" block for this consumer.
+  unset($definitions['help_block']);
+}
+
+/**
+ * @} End of "addtogroup hooks".
+ */
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index 04160432de1..b409c535979 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -239,7 +239,8 @@ class BlockForm extends EntityForm {
     // @todo Allow list of conditions to be configured in
     //   https://www.drupal.org/node/2284687.
     $visibility = $this->entity->getVisibility();
-    foreach ($this->manager->getDefinitionsForContexts($form_state->getTemporaryValue('gathered_contexts')) as $condition_id => $definition) {
+    $definitions = $this->manager->getFilteredDefinitions('block_ui', $form_state->getTemporaryValue('gathered_contexts'), ['block' => $this->entity]);
+    foreach ($definitions as $condition_id => $definition) {
       // Don't display the current theme condition.
       if ($condition_id == 'current_theme') {
         continue;
diff --git a/core/modules/block/src/Controller/BlockLibraryController.php b/core/modules/block/src/Controller/BlockLibraryController.php
index 959c642bbaf..cf5f20a0baf 100644
--- a/core/modules/block/src/Controller/BlockLibraryController.php
+++ b/core/modules/block/src/Controller/BlockLibraryController.php
@@ -101,8 +101,14 @@ class BlockLibraryController extends ControllerBase {
       ['data' => $this->t('Operations')],
     ];
 
+    $region = $request->query->get('region');
+    $weight = $request->query->get('weight');
+
     // Only add blocks which work without any available context.
-    $definitions = $this->blockManager->getDefinitionsForContexts($this->contextRepository->getAvailableContexts());
+    $definitions = $this->blockManager->getFilteredDefinitions('block_ui', $this->contextRepository->getAvailableContexts(), [
+      'theme' => $theme,
+      'region' => $region,
+    ]);
     // Order by category, and then by admin label.
     $definitions = $this->blockManager->getSortedDefinitions($definitions);
     // Filter out definitions that are not intended to be placed by the UI.
@@ -110,8 +116,6 @@ class BlockLibraryController extends ControllerBase {
       return empty($definition['_block_ui_hidden']);
     });
 
-    $region = $request->query->get('region');
-    $weight = $request->query->get('weight');
     $rows = [];
     foreach ($definitions as $plugin_id => $plugin_definition) {
       $row = [];
diff --git a/core/modules/layout_builder/src/Controller/ChooseBlockController.php b/core/modules/layout_builder/src/Controller/ChooseBlockController.php
index 85ea7d83709..d046b4fab82 100644
--- a/core/modules/layout_builder/src/Controller/ChooseBlockController.php
+++ b/core/modules/layout_builder/src/Controller/ChooseBlockController.php
@@ -63,7 +63,10 @@ class ChooseBlockController implements ContainerInjectionInterface {
     $build['#type'] = 'container';
     $build['#attributes']['class'][] = 'block-categories';
 
-    $definitions = $this->blockManager->getDefinitionsForContexts($this->getAvailableContexts($section_storage));
+    $definitions = $this->blockManager->getFilteredDefinitions('layout_builder', $this->getAvailableContexts($section_storage), [
+      'section_storage' => $section_storage,
+      'region' => $region,
+    ]);
     foreach ($this->blockManager->getGroupedDefinitions($definitions) as $category => $blocks) {
       $build[$category]['#type'] = 'details';
       $build[$category]['#open'] = TRUE;
diff --git a/core/modules/layout_builder/src/Controller/ChooseSectionController.php b/core/modules/layout_builder/src/Controller/ChooseSectionController.php
index 488480883d5..3405692e53b 100644
--- a/core/modules/layout_builder/src/Controller/ChooseSectionController.php
+++ b/core/modules/layout_builder/src/Controller/ChooseSectionController.php
@@ -62,7 +62,8 @@ class ChooseSectionController implements ContainerInjectionInterface {
     $output['#title'] = $this->t('Choose a layout');
 
     $items = [];
-    foreach ($this->layoutManager->getDefinitions() as $plugin_id => $definition) {
+    $definitions = $this->layoutManager->getFilteredDefinitions('layout_builder', [], ['section_storage' => $section_storage]);
+    foreach ($definitions as $plugin_id => $definition) {
       $layout = $this->layoutManager->createInstance($plugin_id);
       $item = [
         '#type' => 'link',
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.info.yml b/core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.info.yml
new file mode 100644
index 00000000000..607877e7808
--- /dev/null
+++ b/core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.info.yml
@@ -0,0 +1,6 @@
+name: 'Layout Builder test'
+type: module
+description: 'Support module for testing layout building.'
+package: Testing
+version: VERSION
+core: 8.x
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.module b/core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.module
new file mode 100644
index 00000000000..6d159850e83
--- /dev/null
+++ b/core/modules/layout_builder/tests/modules/layout_builder_test/layout_builder_test.module
@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * @file
+ * Provides hook implementations for Layout Builder tests.
+ */
+
+/**
+ * Implements hook_plugin_filter_TYPE__CONSUMER_alter().
+ */
+function layout_builder_test_plugin_filter_block__layout_builder_alter(array &$definitions) {
+  // Explicitly remove the "Help" blocks from the list.
+  unset($definitions['help_block']);
+
+  // Explicitly remove the "Sticky at top of lists field_block".
+  $disallowed_fields = [
+    'sticky',
+  ];
+
+  foreach ($definitions as $plugin_id => $definition) {
+    // Field block IDs are in the form 'field_block:{entity}:{bundle}:{name}',
+    // for example 'field_block:node:article:revision_timestamp'.
+    preg_match('/field_block:.*:.*:(.*)/', $plugin_id, $parts);
+    if (isset($parts[1]) && in_array($parts[1], $disallowed_fields, TRUE)) {
+      // Unset any field blocks that match our predefined list.
+      unset($definitions[$plugin_id]);
+    }
+  }
+}
diff --git a/core/modules/layout_builder/tests/src/Functional/LayoutBuilderTest.php b/core/modules/layout_builder/tests/src/Functional/LayoutBuilderTest.php
index a78e9eeeb79..b99c273f551 100644
--- a/core/modules/layout_builder/tests/src/Functional/LayoutBuilderTest.php
+++ b/core/modules/layout_builder/tests/src/Functional/LayoutBuilderTest.php
@@ -19,6 +19,7 @@ class LayoutBuilderTest extends BrowserTestBase {
     'layout_test',
     'block',
     'node',
+    'layout_builder_test',
   ];
 
   /**
@@ -319,4 +320,33 @@ class LayoutBuilderTest extends BrowserTestBase {
     $this->clickLink('Cancel Layout');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function testLayoutBuilderChooseBlocksAlter() {
+    // See layout_builder_test_plugin_filter_block__layout_builder_alter().
+    $assert_session = $this->assertSession();
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'configure any layout',
+      'administer node display',
+      'administer node fields',
+    ]));
+
+    // From the manage display page, go to manage the layout.
+    $this->drupalGet('admin/structure/types/manage/bundle_with_section_field/display/default');
+    $this->clickLink('Manage layout');
+
+    // Add a new block.
+    $this->clickLink('Add Block');
+
+    // Verify that blocks not modified are present.
+    $assert_session->linkExists('Powered by Drupal');
+    $assert_session->linkExists('Default revision');
+
+    // Verify that blocks explicitly removed are not present.
+    $assert_session->linkNotExists('Help');
+    $assert_session->linkNotExists('Sticky at top of lists');
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/Core/Plugin/FilteredPluginManagerTraitTest.php b/core/tests/Drupal/Tests/Core/Plugin/FilteredPluginManagerTraitTest.php
new file mode 100644
index 00000000000..9642949de25
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Plugin/FilteredPluginManagerTraitTest.php
@@ -0,0 +1,121 @@
+<?php
+
+namespace Drupal\Tests\Core\Plugin;
+
+use Drupal\Component\Plugin\PluginManagerBase;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\Context\ContextHandlerInterface;
+use Drupal\Core\Plugin\FilteredPluginManagerInterface;
+use Drupal\Core\Plugin\FilteredPluginManagerTrait;
+use Drupal\Core\Theme\ThemeManagerInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\Core\Plugin\FilteredPluginManagerTrait
+ * @group Plugin
+ */
+class FilteredPluginManagerTraitTest extends UnitTestCase {
+
+  /**
+   * @covers ::getFilteredDefinitions
+   * @dataProvider providerTestGetFilteredDefinitions
+   */
+  public function testGetFilteredDefinitions($contexts, $expected) {
+    // Start with two plugins.
+    $definitions = [];
+    $definitions['plugin1'] = ['id' => 'plugin1'];
+    $definitions['plugin2'] = ['id' => 'plugin2'];
+
+    $type = 'the_type';
+    $consumer = 'the_consumer';
+    $extra = ['foo' => 'bar'];
+
+    $context_handler = $this->prophesize(ContextHandlerInterface::class);
+    // Remove the second plugin when context1 is provided.
+    $context_handler->filterPluginDefinitionsByContexts(['context1' => 'fake context'], $definitions)
+      ->willReturn(['plugin1' => $definitions['plugin1']]);
+    // Remove the first plugin when no contexts are provided.
+    $context_handler->filterPluginDefinitionsByContexts([], $definitions)
+      ->willReturn(['plugin2' => $definitions['plugin2']]);
+
+    // After context filtering, the alter hook will be invoked.
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+    $hooks = ["plugin_filter_{$type}", "plugin_filter_{$type}__{$consumer}"];
+    $module_handler->alter($hooks, $expected, $extra, $consumer)->shouldBeCalled();
+
+    $theme_manager = $this->prophesize(ThemeManagerInterface::class);
+    $theme_manager->alter($hooks, $expected, $extra, $consumer)->shouldBeCalled();
+
+    $plugin_manager = new TestFilteredPluginManager($definitions, $module_handler->reveal(), $theme_manager->reveal(), $context_handler->reveal());
+    $result = $plugin_manager->getFilteredDefinitions($consumer, $contexts, $extra);
+    $this->assertSame($expected, $result);
+  }
+
+  /**
+   * Provides test data for ::testGetFilteredDefinitions().
+   */
+  public function providerTestGetFilteredDefinitions() {
+    $data = [];
+    $data['populated context'] = [
+      ['context1' => 'fake context'],
+      ['plugin1' => ['id' => 'plugin1']],
+    ];
+    $data['empty context'] = [
+      [],
+      ['plugin2' => ['id' => 'plugin2']],
+    ];
+    $data['null context'] = [
+      NULL,
+      [
+        'plugin1' => ['id' => 'plugin1'],
+        'plugin2' => ['id' => 'plugin2'],
+      ],
+    ];
+    return $data;
+  }
+
+}
+
+/**
+ * Class that allows testing the trait.
+ */
+class TestFilteredPluginManager extends PluginManagerBase implements FilteredPluginManagerInterface {
+
+  use FilteredPluginManagerTrait;
+
+  protected $definitions = [];
+
+  protected $moduleHandler;
+
+  protected $themeManager;
+
+  protected $contextHandler;
+
+  public function __construct(array $definitions, ModuleHandlerInterface $module_handler, ThemeManagerInterface $theme_manager, ContextHandlerInterface $context_handler) {
+    $this->definitions = $definitions;
+    $this->moduleHandler = $module_handler;
+    $this->themeManager = $theme_manager;
+    $this->contextHandler = $context_handler;
+  }
+
+  protected function contextHandler() {
+    return $this->contextHandler;
+  }
+
+  protected function moduleHandler() {
+    return $this->moduleHandler;
+  }
+
+  protected function themeManager() {
+    return $this->themeManager;
+  }
+
+  protected function getType() {
+    return 'the_type';
+  }
+
+  public function getDefinitions() {
+    return $this->definitions;
+  }
+
+}