“Free” refers to the software license of the source code. That is it.
The maintainers of a free software project don’t even have to accept contributions outside of their organization or club.
Private companies that use all kinds of proprietary communication tools regularly contribute to free software. Are all of Red Hat’s internal conversations about Fedora guaranteed to make it into the public?
People are also perfectly capable of having private conversations about contributions to free software projects. These conversations don’t ever have to be made public. Again, only the code license is what makes a piece of software free.
So if you don’t like a project’s method of communication, my advice would be to not contribute to it. It’s the project’s own risk of deterring potential contributors, not yours.
I find it hilarious that someone would find themselves feeling entitled enough to tell a bunch of unpaid open source developers how to communicate with one another as if that someone were their boss at a company. The only place where I’m told what communication tools to use is at work, where I’m paid to comply.