There's not much point in observing "but you could have done those things with email!". We could have done them with tarballs before git existed, too, if we built sufficient additional tooling atop them. That doesn't mean we have the functionality of current forges in a federated model, yet.
Those exist (badly and not integrated) as part of additional tools such as email, or as tasks done manually, or as part of forge software.
I don't think there's much point in splitting this hair further. I stand by the original statement that I'd love to see federated pull requests between forges, with all the capabilities people expect of a modern forge.
There is code or repository, there is a diff or patch. Everything else your labeling as pull request is unknown, not part of original design, debatable.
GitHub style pull request is not part of the original design. What aspects and features you want to keep, and what exactly you say many others are interested in?
We don't even know what a forge is. Let alone a modern one.