git send-email

I'm not sure how that would work without something around git. It would require push rights to your git instance: you'd need to add a bunch of tooling to protect yourself:

• stop people from pushing junk to your server e.g large files, endless while loop that pushes issues filled with random characters or just counting endlessly
• stop people from pushing malicious stuff that can infect you by running the git hook that checks the content
• ensure you have protected branches (again probably a git hook?)

You'd need notifications that somebody has create and issue and PR, or a web interface around git so you can see it.

radicle has made something that works, but it required a gossip protocol to do a lot of work. There's git-bug, but that also runs into the problem of allowing others access to your git.

A simple standard won't cut it. There is way more that has to be considered besides a simple file format. That's exactly why git-forges exist. I'm not saying it's impossible, but way more difficult than a git forge (IMO).