Context: Symbiotic has an extension for theming that uses this trick to detect whether it's running in a frontend or backend form (to avoid loading our CSS on the frontend):

function adminimore_civicrm_config(&$config) {
  $path = CRM_Utils_System::getUrlPath();
  $item = CRM_Core_Menu::get($path);

  $resources = CRM_Core_Resources::singleton();

  // if item is not known, assume it's public (e.g. wordpress shortcode)
  if ($item && !CRM_Utils_Array::value('is_public', $item)) {
    $resources->addStyleFile(ADMINIMORE_RESOURCE, 'css/civicrm-admin.css', 15, 'html-header');
    $resources->addStyleFile(ADMINIMORE_RESOURCE, 'css/civicrm-buttons.css', 15, 'html-header');
  }
  $resources->addStyleFile(ADMINIMORE_RESOURCE, 'css/civicrm-menu.css', 15, 'html-header');

  _adminimore_civix_civicrm_config($config);
}

In Drupal8, we were having weird problems where the CSS would sometimes not load. If we refreshed, it loaded, but after some time, the bug would appear again and we would stumble on a screen using the default CiviCRM CSS.

After some poking around, it seems that Drupal8 deprecated the 'q' variable, which CiviCRM is still using in CRM_Utils_System::getUrlPath().

I did a quick patch to test a workaround, which so far seems to be working:

  public static function getUrlPath() {
    if (CRM_Core_Config::singleton()->userFramework == 'Drupal8') {
      if (class_exists('Drupal') && \Drupal::hasContainer()) {
        $path = \Drupal::service('path.current')->getPath();

        // Remove '/' prefix. Ex: '/civicrm/contribute' becomes 'civicrm/contribute'.
        if ($path) {
          $path = substr($path, 1);

          // Remove the language prefix, if present
          // The URL returned by Drupal randomly includes the language prefix, sometimes not.
          if (preg_match('/^\w\w\//', $path)) {
            $path = substr($path, 3);
          }

          return $path;
        }
      }
    }

    if (isset($_GET[CRM_Core_Config::singleton()->userFrameworkURLVar])) {
      return $_GET[CRM_Core_Config::singleton()->userFrameworkURLVar];
    }
    return NULL;
  }

A cleaner solution might be to check if the $config->userSystem->getUrlPath() function exists, and if it does, call it?