9 files declare their use of EntityReferenceItem in Zircon Profile 8.0

ConfigurableEntityReferenceItem.php in core/modules/entity_reference/src/ConfigurableEntityReferenceItem.php

Contains \Drupal\entity_reference\ConfigurableEntityReferenceItem.

DefaultSelection.php in core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php

Contains \Drupal\Core\Entity\Plugin\EntityReferenceSelection\DefaultSelection.

EntityReferenceFieldItemNormalizer.php in core/modules/serialization/src/Normalizer/EntityReferenceFieldItemNormalizer.php

Contains \Drupal\serialization\Normalizer\EntityReferenceFieldItemNormalizer.

EntityReferenceFieldItemNormalizerTest.php in core/modules/serialization/tests/src/Unit/Normalizer/EntityReferenceFieldItemNormalizerTest.php

Contains \Drupal\Tests\serialization\Unit\Normalizer\EntityReferenceFieldItemNormalizerTest.

EntityReferenceFormatterBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php

Contains \Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase.

EntityViewsDataTest.php in core/modules/views/tests/src/Unit/EntityViewsDataTest.php

Contains \Drupal\Tests\views\Unit\EntityViewsDataTest.

field.install in core/modules/field/field.install

Install, update and uninstall functions for the field module.

FileFormatterBase.php in core/modules/file/src/Plugin/Field/FieldFormatter/FileFormatterBase.php

Contains \Drupal\file\Plugin\Field\FieldFormatter\FileFormatterBase.

FileItem.php in core/modules/file/src/Plugin/Field/FieldType/FileItem.php

Contains \Drupal\file\Plugin\Field\FieldType\FileItem.