Foss Projects and Proprietary Platforms
Discord and Open Source Projects
Or, hear me out: they, of all people, need to be careful of the amount of time they spend on things like deciding which code hosting site to use and which communication venues to use, because they are already deeply constrained for time. And so they will prioritize things like “free of charge to the project” and “easy to set up” and “already have a lot of people onboarded” far more highly than things like theoretical purity.
Which, these days, basically means GitHub and Discord. If you want to change that, being angry at open-source communities will not accomplish your goal. Providing something that ranks highly on the factors I mentioned will be far more effective.