When I was first exposed to Medium (which was before I started coding), the coolaborative notes and highlights really felt like what I would want a literature github to be.

Just as a thought experiment, it's interesting that I think nothing of forking and maintaining a codebase for personal use, but I can't imagine participating in the writing of stories in the same way. Literature seems to maintain the primacy of the 'maintainer', but if I think about the evolution of storytelling generally, this doesn't seem to have always been the case

There's also https://writing.stackexchange.com/