diff --git a/core/lib/Drupal/Core/Block/BlockManager.php b/core/lib/Drupal/Core/Block/BlockManager.php
index 036933f..3c88bdc 100644
--- a/core/lib/Drupal/Core/Block/BlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockManager.php
@@ -8,7 +8,6 @@
 use Drupal\Core\Plugin\CategorizingPluginManagerTrait;
 use Drupal\Core\Plugin\Context\ContextAwarePluginManagerTrait;
 use Drupal\Core\Plugin\DefaultPluginManager;
-use Drupal\Core\Plugin\PluginFormInterface;
 
 /**
  * Manages discovery and instantiation of block plugins.
@@ -49,12 +48,6 @@ public function __construct(\Traversable $namespaces, CacheBackendInterface $cac
   public function processDefinition(&$definition, $plugin_id) {
     parent::processDefinition($definition, $plugin_id);
     $this->processDefinitionCategory($definition);
-
-    // If no default form is defined and this plugin implements
-    // \Drupal\Core\Plugin\PluginFormInterface, use that for the default form.
-    if (!isset($definition['form']['default']) && is_subclass_of($definition['class'], PluginFormInterface::class)) {
-      $definition['form']['default'] = $definition['class'];
-    }
   }
 
   /**
diff --git a/core/modules/block/src/BlockForm.php b/core/modules/block/src/BlockForm.php
index d4b4c42..bd1366f 100644
--- a/core/modules/block/src/BlockForm.php
+++ b/core/modules/block/src/BlockForm.php
@@ -71,11 +71,11 @@ class BlockForm extends EntityForm {
   protected $contextRepository;
 
   /**
-   * The plugin form manager.
+   * The plugin form factory.
    *
    * @var \Drupal\Core\Plugin\PluginFormFactoryInterface
    */
-  protected $pluginFormManager;
+  protected $pluginFormFactory;
 
   /**
    * Constructs a BlockForm object.
@@ -90,16 +90,16 @@ class BlockForm extends EntityForm {
    *   The language manager.
    * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
    *   The theme handler.
-   * @param \Drupal\Core\Plugin\PluginFormFactoryInterface $plugin_form_manager
-   *   The plugin form manager.
+   * @param \Drupal\Core\Plugin\PluginFormFactoryInterface $plugin_form_factory
+   *   The plugin form factory.
    */
-  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, ContextRepositoryInterface $context_repository, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler, PluginFormFactoryInterface $plugin_form_manager) {
+  public function __construct(EntityManagerInterface $entity_manager, ExecutableManagerInterface $manager, ContextRepositoryInterface $context_repository, LanguageManagerInterface $language, ThemeHandlerInterface $theme_handler, PluginFormFactoryInterface $plugin_form_factory) {
     $this->storage = $entity_manager->getStorage('block');
     $this->manager = $manager;
     $this->contextRepository = $context_repository;
     $this->language = $language;
     $this->themeHandler = $theme_handler;
-    $this->pluginFormManager = $plugin_form_manager;
+    $this->pluginFormFactory = $plugin_form_factory;
   }
 
   /**
@@ -435,7 +435,7 @@ public function getUniqueMachineName(BlockInterface $block) {
    *   The plugin form for the block.
    */
   protected function getPluginForm(BlockPluginInterface $block) {
-    return $this->pluginFormManager->createInstance($block, 'configuration');
+    return $this->pluginFormFactory->createInstance($block, 'configuration');
   }
 
 }
diff --git a/core/modules/block/tests/src/Unit/BlockFormTest.php b/core/modules/block/tests/src/Unit/BlockFormTest.php
index cb4bdc2..9d81555 100644
--- a/core/modules/block/tests/src/Unit/BlockFormTest.php
+++ b/core/modules/block/tests/src/Unit/BlockFormTest.php
@@ -56,11 +56,11 @@ class BlockFormTest extends UnitTestCase {
   protected $contextRepository;
 
   /**
-   * The plugin form manager.
+   * The plugin form factory.
    *
    * @var \Drupal\Core\Plugin\PluginFormFactoryInterface|\Prophecy\Prophecy\ProphecyInterface
    */
-  protected $pluginFormManager;
+  protected $pluginFormFactory;
 
   /**
    * {@inheritdoc}
@@ -79,7 +79,7 @@ protected function setUp() {
       ->method('getStorage')
       ->will($this->returnValue($this->storage));
 
-    $this->pluginFormManager = $this->prophesize(PluginFormFactoryInterface::class);
+    $this->pluginFormFactory = $this->prophesize(PluginFormFactoryInterface::class);
   }
 
   /**
@@ -108,7 +108,7 @@ public function testGetUniqueMachineName() {
       ->method('getQuery')
       ->will($this->returnValue($query));
 
-    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler, $this->pluginFormManager->reveal());
+    $block_form_controller = new BlockForm($this->entityManager, $this->conditionManager, $this->contextRepository, $this->language, $this->themeHandler, $this->pluginFormFactory->reveal());
 
     // Ensure that the block with just one other instance gets the next available
     // name suggestion.
diff --git a/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php b/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
index 7f5c868..56ceba4 100644
--- a/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
+++ b/core/modules/outside_in/src/Block/BlockEntityOffCanvasForm.php
@@ -68,7 +68,7 @@ protected function submitVisibility(array $form, FormStateInterface $form_state)
    * {@inheritdoc}
    */
   protected function getPluginForm(BlockPluginInterface $block) {
-    return $this->pluginFormManager->getFormObject($block, 'offcanvas', 'default');
+    return $this->pluginFormFactory->createInstance($block, 'offcanvas', 'configuration');
   }
 
 }
diff --git a/core/modules/outside_in/src/Form/SystemBrandingOffCanvasForm.php b/core/modules/outside_in/src/Form/SystemBrandingOffCanvasForm.php
index 13f037c..6e71c76 100644
--- a/core/modules/outside_in/src/Form/SystemBrandingOffCanvasForm.php
+++ b/core/modules/outside_in/src/Form/SystemBrandingOffCanvasForm.php
@@ -2,12 +2,10 @@
 
 namespace Drupal\outside_in\Form;
 
-
-use Drupal\Component\Plugin\PluginAwareInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\Plugin\PluginFormBase;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -15,7 +13,7 @@
  *
  * @see outside_in_block_alter()
  */
-class SystemBrandingOffCanvasForm implements ContainerInjectionInterface, PluginAwareInterface, PluginFormInterface {
+class SystemBrandingOffCanvasForm extends PluginFormBase implements ContainerInjectionInterface {
 
   /**
    * The plugin.
@@ -53,13 +51,6 @@ public static function create(ContainerInterface $container) {
   /**
    * {@inheritdoc}
    */
-  public function setPlugin($plugin) {
-    $this->plugin = $plugin;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
     $form = $this->plugin->buildConfigurationForm($form, $form_state);
 
diff --git a/core/modules/outside_in/src/Form/SystemMenuOffCanvasForm.php b/core/modules/outside_in/src/Form/SystemMenuOffCanvasForm.php
index 997d7a8..627d42e 100644
--- a/core/modules/outside_in/src/Form/SystemMenuOffCanvasForm.php
+++ b/core/modules/outside_in/src/Form/SystemMenuOffCanvasForm.php
@@ -2,12 +2,12 @@
 
 namespace Drupal\outside_in\Form;
 
-use Drupal\Component\Plugin\PluginAwareInterface;
+use Drupal\Component\Plugin\PluginInspectionInterface;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\Plugin\PluginFormBase;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Routing\RedirectDestinationTrait;
 use Drupal\Core\StringTranslation\StringTranslationTrait;
@@ -20,7 +20,7 @@
  *
  * @see outside_in_block_alter()
  */
-class SystemMenuOffCanvasForm implements ContainerInjectionInterface, PluginAwareInterface, PluginFormInterface {
+class SystemMenuOffCanvasForm extends PluginFormBase implements ContainerInjectionInterface {
 
   use StringTranslationTrait;
   use RedirectDestinationTrait;
@@ -116,7 +116,11 @@ public function submitConfigurationForm(array &$form, FormStateInterface $form_s
   /**
    * Gets the entity form for this menu.
    *
-   * @return \Drupal\Core\Form\FormInterface
+   * @param \Drupal\system\MenuInterface $entity
+   *   The menu entity.
+   *
+   * @return \Drupal\Core\Entity\EntityFormInterface
+   *   The entity form.
    */
   protected function getEntityForm(MenuInterface $entity) {
     $entity_form = $this->entityTypeManager->getFormObject('menu', 'edit');
@@ -145,12 +149,9 @@ protected function ensureFormState(FormStateInterface $form_state) {
   }
 
   /**
-   * Sets the plugin for this object.
-   *
-   * @param object $plugin
-   *   The plugin.
+   * {@inheritdoc}
    */
-  public function setPlugin($plugin) {
+  public function setPlugin(PluginInspectionInterface $plugin) {
     $this->plugin = $plugin;
     $this->entity = $this->menuStorage->load($this->plugin->getDerivativeId());
   }