addClass to sub ul

menu.html.twig

{{ menus.menu_links(items, attributes, 0, menu_name) }}

{% macro menu_links(items, attributes, menu_level, menu_name) %}
  {% import _self as menus %}
  {%
    set menu_classes = [
      'o-menu',
      'c-menu-' ~ menu_name|clean_class,
    ]
  %}
  {%
    set submenu_classes = [
      'o-menu',
      'c-menu-' ~ menu_name|clean_class ~ '__submenu',
    ]
  %}
  {% if items %}
    {% if menu_level == 0 %}
      <ul{{ attributes.addClass(menu_classes) }}>
    {% else %}
      <ul{{ attributes.removeClass(menu_classes).addClass(submenu_classes) }}>
    {% endif %}