Discord alternatives that I use and like:
IRC: been using it since 1994, it still exists and has continued to work fine the entire time https://libera.chat/ I pay for IRCCloud as my primary client, but use irssi as well.
Zulip: tried this out last year as an alternative to Slack. The threading/topics model takes a bit of adjustment, but it's a pretty smooth experience overall. https://zulip.com/