20 files declare their use of PluginBase in Drupal 9

AccessPluginBase.php in core/modules/views/src/Plugin/views/access/AccessPluginBase.php

ArgumentDefaultPluginBase.php in core/modules/views/src/Plugin/views/argument_default/ArgumentDefaultPluginBase.php

ArgumentValidatorPluginBase.php in core/modules/views/src/Plugin/views/argument_validator/ArgumentValidatorPluginBase.php

CachePluginBase.php in core/modules/views/src/Plugin/views/cache/CachePluginBase.php

DisplayExtenderPluginBase.php in core/modules/views/src/Plugin/views/display_extender/DisplayExtenderPluginBase.php

DisplayPluginBase.php in core/modules/views/src/Plugin/views/display/DisplayPluginBase.php

EntityTranslationRenderTrait.php in core/modules/views/src/Entity/Render/EntityTranslationRenderTrait.php

ExposedFormPluginBase.php in core/modules/views/src/Plugin/views/exposed_form/ExposedFormPluginBase.php

LanguageFilter.php in core/modules/views/src/Plugin/views/filter/LanguageFilter.php

NodeLanguageTest.php in core/modules/node/tests/src/Functional/Views/NodeLanguageTest.php

PagerPluginBase.php in core/modules/views/src/Plugin/views/pager/PagerPluginBase.php

PluginBaseTest.php in core/modules/views/tests/src/Kernel/Plugin/PluginBaseTest.php

Contains \Drupal\views\Tests\Plugin\PluginBaseTest.

PluginInstanceTest.php in core/modules/views/tests/src/Kernel/PluginInstanceTest.php

QueryPluginBase.php in core/modules/views/src/Plugin/views/query/QueryPluginBase.php

RowPluginBase.php in core/modules/views/src/Plugin/views/row/RowPluginBase.php

StylePluginBase.php in core/modules/views/src/Plugin/views/style/StylePluginBase.php

TestHelperPlugin.php in core/modules/views/src/Tests/TestHelperPlugin.php

views.api.php in core/modules/views/views.api.php

Describes hooks and plugins provided by the Views module.

views.views_execution.inc in core/modules/views/views.views_execution.inc

Provides views runtime hooks for views.module.

WizardPluginBase.php in core/modules/views/src/Plugin/views/wizard/WizardPluginBase.php