12 files declare their use of PluginBase in Zircon Profile 8

ContextualLinkDefault.php in core/lib/Drupal/Core/Menu/ContextualLinkDefault.php

Contains \Drupal\Core\Menu\ContextualLinkDefault.

DrupalImageCaption.php in core/modules/ckeditor/src/Plugin/CKEditorPlugin/DrupalImageCaption.php

Contains \Drupal\ckeditor\Plugin\CKEditorPlugin\DrupalImageCaption.

Editor.php in core/modules/editor/src/Plugin/InPlaceEditor/Editor.php

Contains \Drupal\editor\Plugin\InPlaceEditor\Editor.

Llama.php in core/modules/ckeditor/tests/modules/src/Plugin/CKEditorPlugin/Llama.php

Contains \Drupal\ckeditor_test\Plugin\CKEditorPlugin\Llama.

LocalActionDefault.php in core/lib/Drupal/Core/Menu/LocalActionDefault.php

Contains \Drupal\Core\Menu\LocalActionDefault.

LocalTaskDefault.php in core/lib/Drupal/Core/Menu/LocalTaskDefault.php

Contains \Drupal\Core\Menu\LocalTaskDefault.

MockLayoutBlock.php in core/modules/system/tests/modules/plugin_test/src/Plugin/plugin_test/mock_block/MockLayoutBlock.php

Contains \Drupal\plugin_test\Plugin\plugin_test\mock_block\MockLayoutBlock.

MockTestBlock.php in core/modules/system/tests/modules/plugin_test/src/Plugin/plugin_test/mock_block/MockTestBlock.php

Contains \Drupal\plugin_test\Plugin\plugin_test\mock_block\MockTestBlock.

MockUserLoginBlock.php in core/modules/system/tests/modules/plugin_test/src/Plugin/plugin_test/mock_block/MockUserLoginBlock.php

Contains \Drupal\plugin_test\Plugin\plugin_test\mock_block\MockUserLoginBlock.

PluginBase.php in core/lib/Drupal/Core/Plugin/PluginBase.php

Contains \Drupal\Core\Plugin\PluginBase.

QueueWorkerBase.php in core/lib/Drupal/Core/Queue/QueueWorkerBase.php

Contains \Drupal\Core\Queue\QueueWorkerBase.

TestConfigurablePlugin.php in core/tests/Drupal/Tests/Core/Plugin/Fixtures/TestConfigurablePlugin.php

Contains \Drupal\Tests\Core\Plugin\Fixtures\TestConfigurablePlugin.