diff --git a/core/profiles/demo_umami/themes/umami/css/classy/README.txt b/core/profiles/demo_umami/themes/umami/css/classy/README.txt
new file mode 100644
index 00000000000..42514f5f0c7
--- /dev/null
+++ b/core/profiles/demo_umami/themes/umami/css/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for CSS files previously inherited from the Classy theme.
+
+WHY ARE CLASSY CSS FILES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, CSS files that would otherwise be inherited from Classy are copied
+here.
+
+CSS files that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/profiles/demo_umami/themes/umami/templates/classy/README.txt b/core/profiles/demo_umami/themes/umami/templates/classy/README.txt
new file mode 100644
index 00000000000..8708a82e744
--- /dev/null
+++ b/core/profiles/demo_umami/themes/umami/templates/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for templates previously inherited from the Classy theme.
+
+WHY ARE CLASSY TEMPLATES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, templates that would otherwise be inherited from Classy are copied
+here.
+
+Templates that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/themes/bartik/css/classy/README.txt b/core/themes/bartik/css/classy/README.txt
new file mode 100644
index 00000000000..42514f5f0c7
--- /dev/null
+++ b/core/themes/bartik/css/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for CSS files previously inherited from the Classy theme.
+
+WHY ARE CLASSY CSS FILES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, CSS files that would otherwise be inherited from Classy are copied
+here.
+
+CSS files that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/themes/bartik/templates/classy/README.txt b/core/themes/bartik/templates/classy/README.txt
new file mode 100644
index 00000000000..8708a82e744
--- /dev/null
+++ b/core/themes/bartik/templates/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for templates previously inherited from the Classy theme.
+
+WHY ARE CLASSY TEMPLATES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, templates that would otherwise be inherited from Classy are copied
+here.
+
+Templates that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/themes/claro/css/classy/README.txt b/core/themes/claro/css/classy/README.txt
new file mode 100644
index 00000000000..42514f5f0c7
--- /dev/null
+++ b/core/themes/claro/css/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for CSS files previously inherited from the Classy theme.
+
+WHY ARE CLASSY CSS FILES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, CSS files that would otherwise be inherited from Classy are copied
+here.
+
+CSS files that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/themes/claro/templates/classy/README.txt b/core/themes/claro/templates/classy/README.txt
new file mode 100644
index 00000000000..8708a82e744
--- /dev/null
+++ b/core/themes/claro/templates/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for templates previously inherited from the Classy theme.
+
+WHY ARE CLASSY TEMPLATES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, templates that would otherwise be inherited from Classy are copied
+here.
+
+Templates that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/themes/seven/css/classy/README.txt b/core/themes/seven/css/classy/README.txt
new file mode 100644
index 00000000000..42514f5f0c7
--- /dev/null
+++ b/core/themes/seven/css/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for CSS files previously inherited from the Classy theme.
+
+WHY ARE CLASSY CSS FILES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, CSS files that would otherwise be inherited from Classy are copied
+here.
+
+CSS files that differ from the Classy versions should not be placed in this
+directory or any subdirectory.
diff --git a/core/themes/seven/templates/classy/README.txt b/core/themes/seven/templates/classy/README.txt
new file mode 100644
index 00000000000..8708a82e744
--- /dev/null
+++ b/core/themes/seven/templates/classy/README.txt
@@ -0,0 +1,12 @@
+WHAT IS THIS DIRECTORY FOR?
+--------------------------------
+This directory is for templates previously inherited from the Classy theme.
+
+WHY ARE CLASSY TEMPLATES BEING COPIED HERE?
+-------------------------------------------
+Classy will be deprecated during the Drupal 9 lifecycle. To prepare for Classy's
+removal, templates that would otherwise be inherited from Classy are copied
+here.
+
+Templates that differ from the Classy versions should not be placed in this
+directory or any subdirectory.