diff --git a/core/lib/Drupal/Core/Config/Schema/Sequence.php b/core/lib/Drupal/Core/Config/Schema/Sequence.php
index 8e715baf670..66a5c3a36f8 100644
--- a/core/lib/Drupal/Core/Config/Schema/Sequence.php
+++ b/core/lib/Drupal/Core/Config/Schema/Sequence.php
@@ -28,6 +28,8 @@ class Sequence extends ArrayElement {
     $definition = [];
     if (isset($this->definition['sequence'][0])) {
       $definition = $this->definition['sequence'][0];
+      $bc_sequence_location = $this->getPropertyPath();
+      @trigger_error("The definition for the '$bc_sequence_location' sequence declares the type of its items in a way that is deprecated in drupal:8.0.0 and is removed from drupal:11.0.0. See https://www.drupal.org/node/2442603", E_USER_DEPRECATED);
     }
     elseif ($this->definition['sequence']) {
       $definition = $this->definition['sequence'];
diff --git a/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml b/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml
index 4dae8329048..2ef49605513 100644
--- a/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml
+++ b/core/modules/config/tests/config_schema_test/config/schema/config_schema_test.schema.yml
@@ -120,6 +120,7 @@ config_schema_test.schema_data_types:
       type: sequence
       sequence:
         type: boolean
+    # @see \Drupal\Core\Config\Schema\Sequence::getElementDefinition()
     sequence_bc:
       type: sequence
       sequence:
@@ -220,7 +221,7 @@ wrapping.config_schema_test.plugin_types:
     tests:
       type: sequence
       sequence:
-        - type: wrapping.test.plugin_types.[plugin_id]
+        type: wrapping.test.plugin_types.[plugin_id]
 
 wrapping.test.plugin_types.*:
   type: test.plugin_types.[plugin_id]
@@ -240,7 +241,7 @@ wrapping.config_schema_test.double_brackets:
     tests:
       type: sequence
       sequence:
-        - type: wrapping.test.double_brackets.[another_key]
+        type: wrapping.test.double_brackets.[another_key]
 
 wrapping.test.double_brackets.*:
   type: test.double_brackets.[foo].[bar]
@@ -277,7 +278,7 @@ wrapping.config_schema_test.other_double_brackets:
     tests:
       type: sequence
       sequence:
-        - type: wrapping.test.other_double_brackets.[id]
+        type: wrapping.test.other_double_brackets.[id]
 
 wrapping.test.other_double_brackets.*:
   type: test.double_brackets.[id]
@@ -305,36 +306,36 @@ config_schema_test.schema_sequence_sort:
       type: sequence
       orderby: key
       sequence:
-        - type: string
+        type: string
     value_sort:
       type: sequence
       orderby: value
       sequence:
-        - type: string
+        type: string
     no_sort:
       type: sequence
       sequence:
-        - type: string
+        type: string
     complex_sort_value:
       type: sequence
       orderby: value
       sequence:
-        - type: mapping
-          mapping:
-            foo:
-              type: string
-            bar:
-              type: string
+        type: mapping
+        mapping:
+          foo:
+            type: string
+          bar:
+            type: string
     complex_sort_key:
       type: sequence
       orderby: key
       sequence:
-        - type: mapping
-          mapping:
-            foo:
-              type: string
-            bar:
-              type: string
+        type: mapping
+        mapping:
+          foo:
+            type: string
+          bar:
+            type: string
 
 config_schema_test.schema_mapping_sort:
   type: config_object
diff --git a/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php b/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
index 8a0d99bfcde..f5f9df5385f 100644
--- a/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Config/ConfigSchemaTest.php
@@ -352,6 +352,8 @@ class ConfigSchemaTest extends KernelTestBase {
 
   /**
    * Tests configuration value data type enforcement using schemas.
+   *
+   * @group legacy
    */
   public function testConfigSaveWithSchema() {
     $untyped_values = [
@@ -399,6 +401,7 @@ class ConfigSchemaTest extends KernelTestBase {
     ];
 
     // Save config which has a schema that enforces types.
+    $this->expectDeprecation("The definition for the 'config_schema_test.schema_data_types.sequence_bc' sequence declares the type of its items in a way that is deprecated in drupal:8.0.0 and is removed from drupal:11.0.0. See https://www.drupal.org/node/2442603");
     $this->config('config_schema_test.schema_data_types')
       ->setData($untyped_to_typed)
       ->save();