42 files declare their use of Plugin in Drupal 8

Action.php in core/lib/Drupal/Core/Annotation/Action.php

AggregatorFetcher.php in core/modules/aggregator/src/Annotation/AggregatorFetcher.php

AggregatorParser.php in core/modules/aggregator/src/Annotation/AggregatorParser.php

AggregatorProcessor.php in core/modules/aggregator/src/Annotation/AggregatorProcessor.php

AnnotatedClassDiscoveryTest.php in core/tests/Drupal/Tests/Component/Annotation/AnnotatedClassDiscoveryTest.php

AnnotationBridgeDecoratorTest.php in core/tests/Drupal/Tests/Component/Annotation/Plugin/Discovery/AnnotationBridgeDecoratorTest.php

Archiver.php in core/lib/Drupal/Core/Archiver/Annotation/Archiver.php

Block.php in core/lib/Drupal/Core/Block/Annotation/Block.php

CKEditorPlugin.php in core/modules/ckeditor/src/Annotation/CKEditorPlugin.php

Condition.php in core/lib/Drupal/Core/Condition/Annotation/Condition.php

Constraint.php in core/lib/Drupal/Core/Validation/Annotation/Constraint.php

ContextDefinition.php in core/lib/Drupal/Core/Annotation/ContextDefinition.php

DataType.php in core/lib/Drupal/Core/TypedData/Annotation/DataType.php

DisplayVariant.php in core/lib/Drupal/Core/Display/Annotation/DisplayVariant.php

Editor.php in core/modules/editor/src/Annotation/Editor.php

EntityReferenceSelection.php in core/lib/Drupal/Core/Entity/Annotation/EntityReferenceSelection.php

EntityType.php in core/lib/Drupal/Core/Entity/Annotation/EntityType.php

FieldFormatter.php in core/lib/Drupal/Core/Field/Annotation/FieldFormatter.php

FieldWidget.php in core/lib/Drupal/Core/Field/Annotation/FieldWidget.php

Filter.php in core/modules/filter/src/Annotation/Filter.php

HelpSection.php in core/modules/help/src/Annotation/HelpSection.php

ImageEffect.php in core/modules/image/src/Annotation/ImageEffect.php

ImageToolkit.php in core/lib/Drupal/Core/ImageToolkit/Annotation/ImageToolkit.php

ImageToolkitOperation.php in core/lib/Drupal/Core/ImageToolkit/Annotation/ImageToolkitOperation.php

InPlaceEditor.php in core/modules/quickedit/src/Annotation/InPlaceEditor.php

LanguageNegotiation.php in core/modules/language/src/Annotation/LanguageNegotiation.php

Layout.php in core/lib/Drupal/Core/Layout/Annotation/Layout.php

Mail.php in core/lib/Drupal/Core/Annotation/Mail.php

MediaSource.php in core/modules/media/src/Annotation/MediaSource.php

MigrateDestination.php in core/modules/migrate/src/Annotation/MigrateDestination.php

MigrateField.php in core/modules/migrate_drupal/src/Annotation/MigrateField.php

MigrateProcessPlugin.php in core/modules/migrate/src/Annotation/MigrateProcessPlugin.php

MigrateSource.php in core/modules/migrate/src/Annotation/MigrateSource.php

PluginExtended.php in core/modules/system/tests/modules/plugin_test_extended/src/Plugin/Annotation/PluginExtended.php

PluginTest.php in core/tests/Drupal/Tests/Component/Annotation/PluginTest.php

QueueWorker.php in core/lib/Drupal/Core/Annotation/QueueWorker.php

RestResource.php in core/modules/rest/src/Annotation/RestResource.php

SearchPlugin.php in core/modules/search/src/Annotation/SearchPlugin.php

SectionStorage.php in core/modules/layout_builder/src/Annotation/SectionStorage.php

Tip.php in core/modules/tour/src/Annotation/Tip.php

ViewsPluginAnnotationBase.php in core/modules/views/src/Annotation/ViewsPluginAnnotationBase.php

WorkflowType.php in core/modules/workflows/src/Annotation/WorkflowType.php