6 files declare their use of EntityReferenceFieldItemList

ComputedReferenceTestFieldItemList.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/ComputedReferenceTestFieldItemList.php
EntityReferenceItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
EntityReferenceItemSubclass.php in core/modules/media_library/tests/modules/media_library_test/src/Plugin/Field/FieldType/EntityReferenceItemSubclass.php
EntityReferenceUuidItem.php in core/modules/jsonapi/tests/modules/jsonapi_test_field_type/src/Plugin/Field/FieldType/EntityReferenceUuidItem.php
FileFieldItemList.php in core/modules/file/src/Plugin/Field/FieldType/FileFieldItemList.php
MediaLibraryFieldWidgetOpener.php in core/modules/media_library/src/MediaLibraryFieldWidgetOpener.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.