PRs are not optional: there is no way to disable them on GitHub. I can't be sure that this is intentional, but it certainly works out well for them that this is one of many properties which make it quite difficult to migrate away from the platform.
Yikes, the PRs on the Linux repo are quite terrible. At least there's a bot to auto-reply with the correct procedure.
I guess you could make a bot that closes any opened PR with a message that PRs are not accepted on Github and a link to the contribution docs.
There's technically a way[1], but you'd have to do it every 6 months which is not great.
https://docs.github.com/en/communities/moderating-comments-a...