Sorry, but as someone who has not even heard about this protocol before (Wikipedia article), I have to ask: what exactly will this change enable MediaWiki (or our editors) to do? Has this been discussed on-wiki somewhere?
EDIT: Oh, I see there's this: T186061: Evaluate Matrix / Element as the recommended chat system for Wikimedia.
matrix:whatever will work as an external link, both as a free link and when enclosed in single brackets. I'm not aware of any on-wiki discussions, but it seemed reasonable to me given the presence of irc:// and xmpp:.
My use case is supporting non-browser-based clients in Template:Matrix-channel. See the Matrix.org meta page for background; Matrix is one of the best FLOSS alternatives (I'd argue the best FLOSS alternative) for Telegram, Slack etc, and has seen some community use.
Has this been discussed on-wiki somewhere?
I don't think this kind of change needs on-wiki discussion, although admittedly I didn't think about non-bracketed links; that seems slightly annoying (but not any more than some existing protocols like tel: or news:). Maybe the list of protocols recognized in bare URLs should be configurable separately. A quick search only finds one enwiki article which will be affected, though, so I don't think this will cause problems.
Should be mentioned in Tech News, though. Maybe something like Strings starting with <code>matrix:</code> will be recognized as [[m:Matrix.org|Matrix]] URLs by the parser.?