I know about git-send-email. But you still need some kind of website for project info and code (unless you only wanna give out your code on request via email) and a patch/bug tracker if you want your project to be approachable. Larger projects and business users might also want CI.
https://git-scm.com/docs/git-send-email
Git was designed for patches (diffs) to be sent in email. That’s how many people used it before GitHub.