field_read_fields__usage.php

<?php

/**
 * Implements hook_field_extra_fields().
 * Add the code to Registration enabled entities
 */
function my_module_field_extra_fields() {
  $extra = [];
  foreach (entity_get_info() as $entity_type => $entity_info) {
    foreach (array_keys($entity_info['bundles']) as $bundle) {
      // @todo need review on how reliable this is for getting registration references
      $field_instance = field_read_fields([
        'entity_type' => $entity_type,
        'bundle' => $bundle,
        'type' => 'registration',
      ]);

      if (!empty($field_instance)) {
        $extra[$entity_type][$bundle]['display']['registration_sms'] = [
          'label' => t('Registration SMS code'),
          'description' => 'SMS code if user is allowed to see it.',
          'weight' => 0,
        ];
      }
    }
  }

  return $extra;
}