> With Github/lab's model, if you force push your PR, you lose the ability to view its previous state and diff against that.

I'm not sure about Gitlab, but Github has recently added a feature where you can view the diff between the old branch head and new one. But, as far as I'm aware, there's no way to check out the previous branch head from the repo due to a lack of a remote branch pointing to it.

At least git itself provides a range-diff command that allows you do see a diff between the commits between two versions of a given branch.