diff -u b/core/lib/Drupal/Core/Render/ProtectedRenderArray.php b/core/lib/Drupal/Core/Render/ProtectedRenderArray.php
--- b/core/lib/Drupal/Core/Render/ProtectedRenderArray.php
+++ b/core/lib/Drupal/Core/Render/ProtectedRenderArray.php
@@ -64,11 +64,6 @@
   /**
    * {@inheritdoc}
    */
-  public function offsetUnSet($index) {}
-
-  /**
-   * {@inheritdoc}
-   */
   public function append($values) {}
 
   /**
diff -u b/core/lib/Drupal/Core/Render/RenderArrayIterator.php b/core/lib/Drupal/Core/Render/RenderArrayIterator.php
--- b/core/lib/Drupal/Core/Render/RenderArrayIterator.php
+++ b/core/lib/Drupal/Core/Render/RenderArrayIterator.php
@@ -39,11 +39,6 @@
   /**
    * {@inheritdoc}
    */
-  public function offsetUnset($index) {}
-
-  /**
-   * {@inheritdoc}
-   */
   public function append($values) {}
 
   /**
diff -u b/core/tests/Drupal/Tests/Core/Render/ProtectedRenderArrayTest.php b/core/tests/Drupal/Tests/Core/Render/ProtectedRenderArrayTest.php
--- b/core/tests/Drupal/Tests/Core/Render/ProtectedRenderArrayTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/ProtectedRenderArrayTest.php
@@ -50,10 +50,6 @@
     $protected_array['new_key'] = 'new_value';
     $this->assertArrayNotHasKey('new_key', $protected_array);
 
-    // Test that existing keys cannot be unset.
-    unset($protected_array['sub_array']);
-    $this->assertArrayHasKey('sub_array', $protected_array);
-
     // Test that existing values cannot be changed.
     $protected_array['sub_markup']['#markup'] = 'Hello world';
     $this->assertTrue($protected_array['sub_markup']['#markup'] === 'Test');