7 files declare their use of LinkCollection in Drupal 9

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

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

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

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

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

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

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