EntityReferenceIdFormatter.php

<?php

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

use Drupal\Core\Field\FieldItemListInterface;

/**
 * Plugin implementation of the 'entity reference ID' formatter.
 *
 * @FieldFormatter(
 *   id = "entity_reference_entity_id",
 *   label = @Translation("Entity ID"),
 *   description = @Translation("Display the ID of the referenced entities."),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
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;
    }

}