diff --git a/core/modules/block/block.api.php b/core/modules/block/block.api.php
index 5a28bad..de97808 100644
--- a/core/modules/block/block.api.php
+++ b/core/modules/block/block.api.php
@@ -209,6 +209,8 @@ function hook_block_access(\Drupal\block\Entity\Block $block, $operation, \Drupa
   // Example code that would prevent displaying the 'Powered by Drupal' block in
   // a region different than the footer.
   if ($operation == 'view' && $block->getPluginId() == 'system_powered_by_block') {
+    // @deprecated in Drupal 8.0.x and will be removed before 9.0.0. Use
+    // \Drupal\Core\Access\AccessResult::addCacheableDependency instead.
     return AccessResult::forbiddenIf($block->getRegion() != 'footer')->cacheUntilEntityChanges($block);
   }