7 files declare their use of JsonApiSpec in Drupal 9

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

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

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

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

JsonApiSpecTest.php in core/modules/jsonapi/tests/src/Unit/JsonApiSpecTest.php

Relationship.php in core/modules/jsonapi/src/JsonApiResource/Relationship.php

ResourceObject.php in core/modules/jsonapi/src/JsonApiResource/ResourceObject.php