/**
 * Implements hook_field_storage_config_update_forbid().
 */
function {{ machine_name }}_field_storage_config_update_forbid(\Drupal\field\FieldStorageConfigInterface $field_storage, \Drupal\field\FieldStorageConfigInterface $prior_field_storage) {
  if ($field_storage->getTypeProvider() == 'options' && $field_storage->hasData()) {
    // Forbid any update that removes allowed values with actual data.
    $allowed_values = $field_storage->getSetting('allowed_values');
    $prior_allowed_values = $prior_field_storage->getSetting('allowed_values');
    $lost_keys = array_keys(array_diff_key($prior_allowed_values, $allowed_values));
    if (_options_values_in_use($field_storage->getTargetEntityTypeId(), $field_storage->getName(), $lost_keys)) {
      throw new \Drupal\Core\Entity\Exception\FieldStorageDefinitionUpdateForbiddenException("A list field '{$field_storage->getName()}' with existing data cannot have its keys changed.");
    }
  }
}