class AdvancedHelpManager extends DefaultPluginManager {
  …

  /**
   * Search the system for all available help topics.
   * @todo check visibility of the method.
   */
  public function getTopics() {
    $ini = $this->parseHelp();
    return $ini['topics'];
  }
  …
}

$foo = new AdvancedHelpManager();
$bar = $foo->getTopics();

$plugin_manager = Drupal::service('plugin.manager.advanced_help');
$topics = $plugin_manager->getTopics();