9 files declare their use of OffsetPage in JSON:API 8

EntityResource.php in src/Controller/EntityResource.php

EntityResourceTest.php in tests/src/Kernel/Controller/EntityResourceTest.php

JsonApiFunctionalTest.php in tests/src/Functional/JsonApiFunctionalTest.php

JsonApiParamEnhancer.php in src/Routing/JsonApiParamEnhancer.php

JsonApiParamEnhancerTest.php in tests/src/Unit/Routing/JsonApiParamEnhancerTest.php

LinkManager.php in src/LinkManager/LinkManager.php

LinkManagerTest.php in tests/src/Unit/LinkManager/LinkManagerTest.php

OffsetPageNormalizer.php in src/Normalizer/OffsetPageNormalizer.php

OffsetPageNormalizerTest.php in tests/src/Kernel/Normalizer/OffsetPageNormalizerTest.php