11 files declare their use of BaseFieldOverride in Drupal 9

BaseFieldDefinition.php in core/lib/Drupal/Core/Field/BaseFieldDefinition.php

BaseFieldOverrideResourceTestBase.php in core/tests/Drupal/FunctionalTests/Rest/BaseFieldOverrideResourceTestBase.php

BaseFieldOverrideTest.php in core/modules/jsonapi/tests/src/Functional/BaseFieldOverrideTest.php

BaseFieldOverrideTest.php in core/tests/Drupal/KernelTests/Core/Field/Entity/BaseFieldOverrideTest.php

ContentTranslationSettingsTest.php in core/modules/content_translation/tests/src/Functional/ContentTranslationSettingsTest.php

MigrateNodeBundleSettingsTest.php in core/modules/node/tests/src/Kernel/Migrate/d6/MigrateNodeBundleSettingsTest.php

MigrateNodeSettingPromoteTest.php in core/modules/node/tests/src/Kernel/Migrate/d6/MigrateNodeSettingPromoteTest.php

MigrateNodeSettingStatusTest.php in core/modules/node/tests/src/Kernel/Migrate/d6/MigrateNodeSettingStatusTest.php

MigrateNodeSettingStickyTest.php in core/modules/node/tests/src/Kernel/Migrate/d6/MigrateNodeSettingStickyTest.php

MigrateNodeTitleLabelTest.php in core/modules/node/tests/src/Kernel/Migrate/d7/MigrateNodeTitleLabelTest.php

NodeFieldOverridesTest.php in core/modules/node/tests/src/Kernel/NodeFieldOverridesTest.php