13 files declare their use of FieldPluginBase in Drupal 9

ContextualLinks.php in core/modules/contextual/src/Plugin/views/field/ContextualLinks.php

DblogMessage.php in core/modules/dblog/src/Plugin/views/field/DblogMessage.php

DblogOperations.php in core/modules/dblog/src/Plugin/views/field/DblogOperations.php

EntityLink.php in core/modules/comment/src/Plugin/views/field/EntityLink.php

FieldFormButtonTest.php in core/modules/views/tests/modules/views_test_data/src/Plugin/views/field/FieldFormButtonTest.php

FieldKernelTest.php in core/modules/views/tests/src/Kernel/Handler/FieldKernelTest.php

FieldPluginBaseTest.php in core/modules/views/tests/src/Unit/Plugin/field/FieldPluginBaseTest.php

Contains \Drupal\Tests\views\Unit\Plugin\field\FieldPluginBaseTest.

FieldTest.php in core/modules/views/tests/modules/views_test_data/src/Plugin/views/field/FieldTest.php

File.php in core/modules/file/src/Plugin/views/field/File.php

MediaLibrarySelectForm.php in core/modules/media_library/src/Plugin/views/field/MediaLibrarySelectForm.php

Node.php in core/modules/node/src/Plugin/views/field/Node.php

StatisticsLastCommentName.php in core/modules/comment/src/Plugin/views/field/StatisticsLastCommentName.php

UserData.php in core/modules/user/src/Plugin/views/field/UserData.php