diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -18,7 +18,9 @@
 /**
  * Defines the theme registry service.
  *
- * @internal
+ * @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.
  */