13 files declare their use of ResourceObject in Drupal 9

EntityAccessChecker.php in core/modules/jsonapi/src/Access/EntityAccessChecker.php

EntityReferenceFieldNormalizer.php in core/modules/jsonapi/src/Normalizer/EntityReferenceFieldNormalizer.php

EntityResource.php in core/modules/jsonapi/src/Controller/EntityResource.php

FieldNormalizer.php in core/modules/jsonapi/src/Normalizer/FieldNormalizer.php

FileUpload.php in core/modules/jsonapi/src/Controller/FileUpload.php

IncludeResolver.php in core/modules/jsonapi/src/IncludeResolver.php

LinkCollectionNormalizerTest.php in core/modules/jsonapi/tests/src/Kernel/Normalizer/LinkCollectionNormalizerTest.php

RelationshipNormalizerTest.php in core/modules/jsonapi/tests/src/Kernel/Normalizer/RelationshipNormalizerTest.php

ResourceObjectNormalizationCacher.php in core/modules/jsonapi/src/EventSubscriber/ResourceObjectNormalizationCacher.php

ResourceObjectNormalizer.php in core/modules/jsonapi/src/Normalizer/ResourceObjectNormalizer.php

ResourceObjectNormalizerCacherTest.php in core/modules/jsonapi/tests/src/Kernel/EventSubscriber/ResourceObjectNormalizerCacherTest.php

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

SerializerTest.php in core/modules/jsonapi/tests/src/Kernel/Serializer/SerializerTest.php