ConfigurableEntityReferenceItem.php

<?php

namespace Drupal\entity_reference;

use Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem;

/**
 * Deprecated. Alternative implementation of the 'entity_reference' field type.
 *
 * @deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Use
 *   \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem instead.
 *
 * @see \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem
 */
class ConfigurableEntityReferenceItem extends EntityReferenceItem {

}