My security collective is honestly considering going back to IRC.
It's becoming increasingly apparent that if you don't use something truly free and open source and host it yourself, you're just setting yourself up for more of this sort of thing.
You can't trust anyone to properly handle the problem of "how the hell do we keep creeps the f*ck away from kids?" with any amount of common sense.
Even if you self-host matrix there are still multiple ways you could be liable for content you don't even know exists. Especially the last 4 points here:
https://telegra.ph/why-not-matrix-08-07
There are even custom message/media types that people use to upload hidden content you can't see even if you're joined to the same channel using a typical client.
I was on yahoo chat as an 11 year old and I was... fine?