D8 menu item expanded by default

MYMODULE.module

/**
 * Implements hook_ENTITY_TYPE_presave().
 *
 * Force the expanded state on menu items in the main menu.
 */
function MYMODULE_menu_link_content_presave(MenuLinkContentInterface $entity) {
  if ($entity->get('menu_name')->value == 'main') {
    $entity->get('expanded')->set(0, TRUE);
  }
}