So strange they chose Slack and Discord when they had a perfectly fine IRC server (that now is deprecated). Instead of trying to improve IRC (either the protocol or any clients), they chose to go with closed-source hosted platforms. Damn shame.
It pops right up for me with #/room/#rust:matrix.org at the top of the list.
Now try searching for Firefox.
There's a Russian Firefox community on Page 2 of the search results. There are beta and nightly groups on page 3. There's various other noise. But the maintainers have hidden the real Matrix room for Firefox so well, it doesn't even seem to be in the search index.
The choice was public. The move has not been public at all.
It is not deprecated. It is completely dead. Try connecting to irc.mozilla.org — you can’t because it’s permanently offline.