EntityTestReferenceCustomCacheTagFormatter.php

<?php

namespace Drupal\entity_test\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\Plugin\Field\FieldFormatter\EntityReferenceFormatterBase;

/**
 * Plugin implementation of the 'entity_reference_custom_cache_tag' formatter.
 *
 * @FieldFormatter(
 *   id = "entity_reference_custom_cache_tag",
 *   label = @Translation("Custom cache tag"),
 *   field_types = {
 *     "entity_reference"
 *   }
 * )
 */
class EntityTestReferenceCustomCacheTagFormatter extends EntityReferenceFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        return [
            '#cache' => [
                'tags' => [
                    'custom_cache_tag',
                ],
            ],
        ];
    }

}