23 files declare their use of PluginBase in Drupal 9

BlockContentDeletionTest.php in core/modules/block_content/tests/src/Kernel/BlockContentDeletionTest.php

BlockContentDeriverTest.php in core/modules/block_content/tests/src/Kernel/BlockContentDeriverTest.php

ConstraintFactoryTest.php in core/tests/Drupal/KernelTests/Core/Validation/ConstraintFactoryTest.php

ContentEntityTest.php in core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php

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

DefaultSingleLazyPluginCollectionTest.php in core/tests/Drupal/Tests/Core/Plugin/DefaultSingleLazyPluginCollectionTest.php

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

EntityReferenceTranslationDeriver.php in core/modules/migrate_drupal/src/Plugin/migrate/EntityReferenceTranslationDeriver.php

EntityTestDefaultPlugin.php in core/modules/system/tests/modules/entity_test/src/Plugin/Validation/Constraint/EntityTestDefaultPlugin.php

ExtraFieldBlockDeriver.php in core/modules/layout_builder/src/Plugin/Derivative/ExtraFieldBlockDeriver.php

FieldBlockDeriver.php in core/modules/layout_builder/src/Plugin/Derivative/FieldBlockDeriver.php

LayoutBuilderEntityViewDisplay.php in core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php

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

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

MigrationPluginManager.php in core/modules/migrate/src/Plugin/MigrationPluginManager.php

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

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

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

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

PluginWithFormsTraitTest.php in core/tests/Drupal/Tests/Core/Plugin/PluginWithFormsTraitTest.php

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

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

WorkflowTypeBase.php in core/modules/workflows/src/Plugin/WorkflowTypeBase.php