Add a class to a Twig link item in Drupal 8

  {% set link_attributes = create_attribute().addClass('navbar-item') %}
  {% if items %}
    {% for item in items %}
        {{ link(item.title, item.url, link_attributes) }}
    {% endfor %}
  {% endif %}

Simplified/abstracted from https://gitlab.com/agaric/patternlibrary/blob/master/agaric/templates/navigation/menu.html.twig

Thanks to a comment on a somewhat related article:

Submitted by gambry (not verified) on Thu, 06/14/2018 - 15:17

Also very useful and apparently not easily findable “How to add attributes to a link”: {% set attributes = attributes.addClass(‘button’) %} {{ link(link_text, link_url, attributes) }}

From https://befused.com/drupal/twig-addclass