For a better understanding of product management guidance for Wikimedia Foundation's focus of Vue.js this task should collect an overview of current usage of OOUI.
Based on extension.json/skin.json oojs-ui* modules and dev experience.

MediaWiki core

• Interfaces
  • HistoryAction.php, DeleteAction.php, action=purge T100161
  • Vast majority of special pages (42 converted, 8 remaining, only prominent non-converted is Special:UserLogin/Special:CreateAccount) T107037
  • Highest evolved interface RCFilters
• 24 Core widgets (from AbandonEditDialog to UsersMultiselectWidget)

Extensions T122014

Skins