10 files declare their use of PluginManagerBase in Drupal 9

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

EntityDisplayFormBase.php in core/modules/field_ui/src/Form/EntityDisplayFormBase.php

FieldLayoutEntityFormDisplayEditForm.php in core/modules/field_layout/src/Form/FieldLayoutEntityFormDisplayEditForm.php

FieldLayoutEntityViewDisplayEditForm.php in core/modules/field_layout/src/Form/FieldLayoutEntityViewDisplayEditForm.php

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

MockBlockManager.php in core/modules/system/tests/modules/plugin_test/src/Plugin/MockBlockManager.php

PluginManagerBaseTest.php in core/tests/Drupal/Tests/Component/Plugin/PluginManagerBaseTest.php

StubFallbackPluginManager.php in core/tests/Drupal/Tests/Component/Plugin/StubFallbackPluginManager.php

StubPluginManagerBaseWithMapper.php in core/tests/Drupal/Tests/Component/Plugin/StubPluginManagerBaseWithMapper.php

TestPluginManager.php in core/modules/system/tests/modules/plugin_test/src/Plugin/TestPluginManager.php