IRC is very low bandwidth & offers the minimal requirements for communication which can be accessible/acceptable for many projects. If something heavier is needed or wanted, XMPP MUCs & Matrix Spaces may be a good option since they are federated & decentralized (although Matrix has unfortunate defacto centralization around Matrix.org, because it requires quite a lot of resources to self-host in both the Python server as well as mirroring all content for all users for its take on federation). Mattermost & Zulip are fine, but require an account (I believe) to the central server but are FOSS & used enough places to be considered stable/trustworthy.
All options can by bridged to all other options (even the proprietary ones) in some manner, but if possible, the defacto server would be FOSS & owned/operated by the community so that that community can define their ToS and/or CoC. This way they are in control of the community rather than requiring users agree to someone else’s—especially a for-profit US corporation’s—terms in order to participate. Some users will want privacy, anonymity, control of personal data, or to get around a firewall/sanctions …and these desires should be considered acceptable. If not self-hosted (requires time/money), it’s still better to choose something using open protocols, like a space on Matrix.org, a big chatroom on XMPP’s Blabber/Conversations, Libera.Chat/OFTC, etc.
I'd second the recommendation for Zulip. It's pretty similar to Slack/Discord, but unlike those it has good support for making the archives public. It also has much better threading support, which is a nice bonus.
It’s expensive to self-host, and centralized if using Matrix.org. It has its uses, but XMPP MUCs have a lot of overlapping features & Prosody/ejabbard can run on a potato by comparison.
It might be expensive for an organization to host, but for individual / small groups it can be as low as $6/mo. I've been running a small Matrix server on a DigitalOcean droplet for years now, and the $6/mo plan has been working fine for 5 users, and three bridges running on that server.
I believe all messages/multimedia from all users & the history all the rooms & private messages need to be mirrored which can end up being a lot even with a few users depending on what they’ve joined. If my understand is wrong tho, I’d like to know so I’m not running around with this notion.
All options can by bridged to all other options (even the proprietary ones) in some manner, but if possible, the defacto server would be FOSS & owned/operated by the community so that that community can define their ToS and/or CoC. This way they are in control of the community rather than requiring users agree to someone else’s—especially a for-profit US corporation’s—terms in order to participate. Some users will want privacy, anonymity, control of personal data, or to get around a firewall/sanctions …and these desires should be considered acceptable. If not self-hosted (requires time/money), it’s still better to choose something using open protocols, like a space on Matrix.org, a big chatroom on XMPP’s Blabber/Conversations, Libera.Chat/OFTC, etc.