[toastal]

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.