diff --git a/core/lib/Drupal/Core/Block/BlockManager.php b/core/lib/Drupal/Core/Block/BlockManager.php
index 8230b24142..797dd53b84 100644
--- a/core/lib/Drupal/Core/Block/BlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockManager.php
@@ -68,6 +68,7 @@ public function processDefinition(&$definition, $plugin_id) {
     parent::processDefinition($definition, $plugin_id);
     $this->processDefinitionCategory($definition);
 
+    // @todo Remove this check in https://www.drupal.org/node/3167432.
     $class = DefaultFactory::getPluginClass($plugin_id, $definition);
     $build_method = new \ReflectionMethod($class, 'build');
     if (!$build_method->hasReturnType() || $build_method->getReturnType()->getName() !== 'array') {
diff --git a/core/lib/Drupal/Core/Block/BlockPluginInterface.php b/core/lib/Drupal/Core/Block/BlockPluginInterface.php
index e4b330517b..e9c8028b99 100644
--- a/core/lib/Drupal/Core/Block/BlockPluginInterface.php
+++ b/core/lib/Drupal/Core/Block/BlockPluginInterface.php
@@ -72,6 +72,8 @@ public function access(AccountInterface $account, $return_as_object = FALSE);
    * @return array
    *   A renderable array representing the content of the block.
    *
+   * @todo Add array as the return type in https://www.drupal.org/node/3167432.
+   *
    * @see \Drupal\block\BlockViewBuilder
    */
   public function build();