The third line was my focus.

There is no such thing as neccesary evil.

You can use different kind of IM solutions depending on the needs for managing a FLOSS project and these must be FLOSS if there is option (by now, always are).

Not every people need voice or video channels. To be exact, I don't know a single FLOSS project which use them, just a daily videocall if they work with SCRUM for the Daily SCRUM which can be made in several platforms.

You can just do this with a group of Matrix rooms (with space or just name prefix like use to be made on IRC) and Jitsi.

As for the code hosting... Is there a problem with just mirroring?

If you use Git you must know that is decentralized and all you need is an account at the hosting service which could be just raw username and password (plus email verification sometimes) or you can just login/register with your account from other service.