admin_toolbar_content-1.0.0/admin_toolbar_content.links.menu.yml

admin_toolbar_content.settings:
  title: 'Admin Toolbar Content'
  route_name: admin_toolbar_content.settings
  menu_name: admin
  parent: system.admin_config_ui

admin_toolbar_content.menu_links:
  deriver: \Drupal\admin_toolbar_content\Plugin\Derivative\AdminToolbarContentMenuLinks