class EntityReferenceTaxonomyTermRssFormatter

class EntityReferenceTaxonomyTermRssFormatter extends EntityReferenceFormatterBase {
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $parent_entity = $items->getEntity();
    $elements = [];
    foreach ($this->getEntitiesToView($items, $langcode) as $entity) {
      $parent_entity->rss_elements[] = [
        'key' => 'category',
        'value' => $entity->label(),
        'attributes' => [
          'domain' => $entity->id() ? Url::fromRoute('entity.taxonomy_term.canonical', [
            'taxonomy_term' => $entity->id(),
          ], [
            'absolute' => TRUE,
          ])
            ->toString() : '',
        ],
      ];
    }
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    // This formatter is only available for taxonomy terms.
    return $field_definition->getFieldStorageDefinition()
      ->getSetting('target_type') == 'taxonomy_term';
  }

}