ui_icons-1.0.x-dev/modules/ui_icons_library/ui_icons_library.links.menu.yml

ui_suite.index:
  title: "UI libraries"
  description: "Design system libraries."
  route_name: ui_suite.index
  parent: system.themes_page

ui_icons_library.index:
  title: 'Icons'
  description: 'Browse icons by packs.'
  route_name: ui_icons_library.index
  parent: ui_suite.index