D8/9 Compatibility

That doesn't mean that it hasn't been completely converted. Just because it happens to work so far doesn't mean there isn't something broken. Is the QueueSettings.php file unused then? Non-existent functions shouldn't be used in the project, so whether it's working or not, this piece needs to be cleaned up.

When I open this module in VS Code, files from this module are full of errors and standards violations. Here's a piece:

  private function get_queues() {            // Unused private function.
    if (!isset(self::$queues)) {
      $queues = array();
      foreach (module_implements('cron_queue_info') as $module) {   // Undefined function module_implements
        $items = module_invoke($module, 'cron_queue_info');             // Undefined function module_invoke
        if (is_array($items)) {
          foreach ($items as &$item) {
            $item['module'] = $module;
          }
          $queues += $items;
        }
      }
      drupal_alter('cron_queue_info', $queues);   // Undefined function drupal_alter
      self::$queues = $queues;
    }
    return $queues;
  }

  /**
   * Implements hook_cronapi().
   */
  public function cronapi() {
    $items = array(); // Short array syntax must be used to define arrays (Drupal.Arrays.DisallowLongArraySyntax.Found)
    if (!variable_get($this->key . '_enabled', TRUE)) {    // Undefined function variable_get
      return $items;
    }

    // Grab the defined cron queues.
    $queues = self::get_queues();    // Non static method 'get_queues' should not be called statically.