Brave has their own built-in adblocker so I don't see them putting any effort into keeping Mv3 around after it's actually removed from Chromium.
You mean MV2. I've said we'll keep support uBO and uMatrix uses of it, at least. This means we'd have support from the maintainers for their builds to produce extensions we can add to our component updater as optional for our users. We are discussing this now with uBO/uM maintainers.