diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 0167a74..d413003 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -18,6 +18,10 @@
 /**
  * Defines the theme registry service.
  *
+ * @internal Expected to be used only internally since every hook_theme()
+ *   implementation depends on the way this class is built. This class may get
+ *   new features in minor releases so this class should be considered final.
+ *
  * @todo Replace local $registry variables in methods with $this->registry.
  */
 class Registry implements DestructableInterface {