EntityReferenceIdFormatter.php

<?php

namespace Drupal\Core\Field\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\Attribute\FieldFormatter;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Plugin implementation of the 'entity reference ID' formatter.
 */
class EntityReferenceIdFormatter extends EntityReferenceFormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
      if ($entity->id()) {
        $elements[$delta] = [
          '#plain_text' => $entity->id(),
          // Create a cache tag entry for the referenced entity. In the case
          // that the referenced entity is deleted, the cache for referring
          // entities must be cleared.
'#cache' => [
            'tags' => $entity->getCacheTags(),
          ],
        ];
      }
    }
    return $elements;
  }

}