Sorting comments from newest to oldest
As CommentStorage::loadThread adds comment_filter tag to its query, you may use hook_query_TAG_alter hook to change comments order:

/**
 * Implements hook_query_TAG_alter() for comment_filter tag.
 *
 * @see CommentStorage::loadThread().
 */
function mymodule_query_comment_filter_alter(Drupal\Core\Database\Query\AlterableInterface $query) {
  // Change comment order to DESC for 'comment' field.
  if ($query-&gt;getMetaData('field_name') == 'comment') {
    /** @var \Drupal\Core\Database\Query\SelectInterface $query */
    $order_by = &amp;$query-&gt;getOrderBy();
    // 'c.cid' is for flat comment lists.
    if (isset($order_by['c.cid']) &amp;&amp; $order_by['c.cid'] == 'ASC') {
      $order_by['c.cid'] = 'DESC';
    }
    // 'torder' is for threated comment lists.
    if (isset($order_by['torder']) &amp;&amp; $order_by['torder'] == 'ASC') {
      $order_by['torder'] = 'DESC';
    }
  }
}