I was about to mention Keybase, but since there is no self-hosting option, I'm afraid I can't recommend them as a communication tool for free software projects. IRC is still suitable and used by some but it is viewed as an prehistoric option. So are there any modern alternatives?
Sort of, there's Jami [0] (formely GNU/Ring) which is actually free software and looks nice, but I haven't tried it yet. Another option is the Matrix protocol [1] and some of its clients like Riot.im [2] fit this free software criteria.