I didn't found a ticket about the future of OOUI and MediaWiki UI. On MediaWiki wiki I can read:
Codex's toolkit will replace OOUI as the default user interface library. Currently development at the Wikimedia Foundation is focused on Codex, the design system for Wikimedia. It features guidelines and a set of tools: design tokens, components, and icons.
New OOUI feature development is limited only to products that are bound to it and have not (yet) been adapted to use Codex instead. —https://mediawiki.org/wiki/OOUI
Is Codex still not enough mature? When Codex will be fully features complete, have you any plans for OOUI and MediaWikiUI? OOUI will be kept in maintenance mode for a moment, but MediaWikiUI will be removed?
This is only a ticket for long term community planification. I am not here to argue that something should be X and not Y. We just need visibility on the matter.