flag-8.x-4.x-dev/tests/modules/flag_link_test/flag_link_test.routing.yml

flag_link_test.page:
  path: '/flag-link-test/{entity_type_id}/{entity_id}/{flag}'
  defaults:
    _controller: \Drupal\flag_link_test\Controller\FlagLinkTest::page
    _title: Flag link test page
  options:
    parameters:
      flag:
        type: entity:flag
  requirements:
    _access: 'TRUE'