How to add custom classes for menu links in Drupal 8 & 9
Example How to add custom classes for menu links in Drupal 8 & 9:
Method 1:
{{ link(item.title, item.url.setOption('attributes', {'class' : 'main-nav__link'})) }}
Method 2:
{{ link(item.title, item.url, { 'class':['main-nav__link']}) }}