OOUI is reasonably stable and can be used in gadgets, if you wish to.
There is some high-level documentation and several examples at
https://www.mediawiki.org/wiki/OOjs_UI and complete API reference at
https://doc.wikimedia.org/oojs-ui/master/ .
Breaking changes still can sometimes happen and when they do, they are not
being announced very widely (they're just committed with "BREAKING CHANGE"
in the subject line). If you start using OOUI and something that used to
work suddenly breaks, please complain at #mediawiki-visualeditor.
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Bartosz Dziewoński