It's just too hard to moderate a space with so little friction, and any friction you add chases away all but the most dedicated users -- and the most dedicated users are often the ones more likely to get entangled in some insane drama and try to burn the whole place to the ground.
It's a difficult problem. I've always wondered what it would actually cost to actually, properly moderate a reasonably sized forum if you paid a professional mod team real wages and gave them proper tools. Probably way more than we would guess.
Fast forward to 2020 when Blizzard put out WoW Classic (basically the original 2004 state of the game again, as a nostalgia trip). I was bummed to find that all discussion of the game had moved into discord servers. And not just one. There was a separate discord for every single class (mage, warrior, priest, etc). Sometimes more than one if the community couldn't agree on which was best. Every guild had their own discord. Special purpose servers existed for niche topics. If you wanted to find a piece of information, you had to hope that a helpful moderator had pinned it somewhere, or else rely on a crappy search feature. If a server is shut down or you get banned, all of that info is lost forever. It's a nightmare.
Discord is a perfectly good tool for real time chat. It is a TERRIBLE tool for summarizing and preserving knowledge. But unfortunately it's increasingly being used for that purpose and I do not for the life of me understand why.
Honestly, I find it easier to check a Discord server than to get useful results from DuckDuckGo these days.
Google has done whatever it takes to incentivize people not to use competitors. It sucks.
I know Telegram absolutely requires a mobile number, though
Some chat apps like Zulip have the option to open a channel to the web. It can even be indexed by search engine bots
But as I mentioned in another comment, what's more important is how easy it is to administer and setup. The experience for site/community owners is the critical factor for adaption.
Gitter exists, and they use Element. As well as many other open source alternatives (including IRC, but I can understand the apprehension with nicserv and all that ceremony).
I think Discord is overkill for my requirements. But I still want a (free) venue (which is not GitHub) where people can ask questions and - maybe, just maybe - form a community around the library. I keep staring at PhpBB ... but it feels too oldskool, so: nope.
I am beginning to like the idea of a self-hosted Discourse[1] thing; there seems to be a fair number of active tech-related communities... maybe if I have some time over Easter I'll investigate further.
Ironically internally we still use Discord for comms.
All about the right tool for the right job.