The literal project we are discussing is just code. It's literally just code. It doesn't have issues, PRs are disabled as much as they can be (by a GitHub action that automatically closes all PRs with a note that code should be submitted elsewhere), and all "other stuff" is disabled.
What you are referring to is more of a mirror-like approach usage of GitHub.
Some big repos or organizations might be able to pull this off, but good luck having a small project and then directing users to go through all of those hoops to submit issues somewhere else, open PRs somewhere else, etc.
The literal project we are discussing is just code. It's literally just code. It doesn't have issues, PRs are disabled as much as they can be (by a GitHub action that automatically closes all PRs with a note that code should be submitted elsewhere), and all "other stuff" is disabled.
https://github.com/mozilla-firefox/firefox