19 files declare their use of FieldPluginBase in Drupal 9

D6FileField.php in core/modules/migrate_drupal/tests/modules/migrate_field_plugin_manager_test/src/Plugin/migrate/field/D6FileField.php

D6NoCoreVersionSpecified.php in core/modules/migrate_drupal/tests/modules/migrate_field_plugin_manager_test/src/Plugin/migrate/field/D6NoCoreVersionSpecified.php

DateField.php in core/modules/datetime/src/Plugin/migrate/field/DateField.php

Email.php in core/modules/field/src/Plugin/migrate/field/Email.php

EntityReference.php in core/modules/field/src/Plugin/migrate/field/d7/EntityReference.php

FieldLeft.php in core/modules/migrate_drupal/tests/modules/migrate_state_finished_test/src/Plugin/migrate/field/FieldLeft.php

FieldRight.php in core/modules/migrate_drupal/tests/modules/migrate_state_finished_test/src/Plugin/migrate/field/FieldRight.php

FileField.php in core/modules/file/src/Plugin/migrate/field/d6/FileField.php

ImageField.php in core/modules/image/src/Plugin/migrate/field/d7/ImageField.php

LinkField.php in core/modules/link/src/Plugin/migrate/field/d6/LinkField.php

ListField.php in core/modules/options/src/Plugin/migrate/field/d7/ListField.php

NumberField.php in core/modules/field/src/Plugin/migrate/field/d7/NumberField.php

OptionsField.php in core/modules/options/src/Plugin/migrate/field/d7/OptionsField.php

OptionWidgetsField.php in core/modules/options/src/Plugin/migrate/field/d6/OptionWidgetsField.php

PhoneField.php in core/modules/telephone/src/Plugin/migrate/field/d7/PhoneField.php

TaxonomyTermReference.php in core/modules/taxonomy/src/Plugin/migrate/field/TaxonomyTermReference.php

TelephoneField.php in core/modules/telephone/src/Plugin/migrate/field/d7/TelephoneField.php

TextField.php in core/modules/text/src/Plugin/migrate/field/d6/TextField.php

TextField.php in core/modules/text/src/Plugin/migrate/field/d7/TextField.php