At some point, a professor was retiring, and he had about 200 issues printed of the "ACM Transactions on Graphics", an entire book shelf. I asked and got it. The years he had were the 70's and 80's, when 3D graphics were research and transitioned from "how to render a line" to "stochastic motion blurred hyper-surface photon tracing". I used to read them as entertainment. Amazing stuff.
None of those are anything like Discord. Mastodon and email aren't even realtime and don't have a voice chat. When I asked for an alternative, I was asking for a suitable substitute for how Discord is actually used. Until the open-source community produces an actual viable alternative, people will continue to use Discord.