Lack of voice chat, message history, media hosting, screen sharing, or just about any other feature most people actually want these days. The people who want a live, purely text-only service are a stark minority anymore.

Might as well suggest smoke signals. IRC is way past its prime. Matrix and modern protocols exist.

Anti Commercial-AI license

XMPP would be the best choice at this point in time, as it doesn't suffer the downfalls of Matrix (though even Matrix would be better than Discord).

I've looked at revolt and well, when I tried using it it was filled with less than reputable individuals. I don't remember specifics but I remember noping the fuck off revolt real quick. If you want text chat, try matrix, if you want voice well, then idk what to say other than discord, team speak or guilded. Guilded is pretty cool, I actually like the client way better than discord but my friends all use discord and guilded isn't a huge platform. It is backed by the Roblox company so you at least know it's gonna be supported, but then again, it's the Roblox company...

Same. Matrix is great.

After seeing a team of fedi software developers drop their Matrix bridge to their Discord after the total lack of moderation tooling resulted in an extremely transphobic spam wave, I for one am not surprised.

Another team I'm aware of also dropped Matrix for other reasons, but went for Zulip instead, which is also open, but more collaboration oriented a la Slack rather than community oriented like Discord, which probably would not fit what the group in the OP is looking for.