I think the point the GP was trying to make is that the GitHub UI ought to be able to allow you to submit a branch with multiple well-organized commits and review each commit separately with its own PR. The curation of the commits that you'd do for stacked PRs could just as easily be done with commits on a single branch; some of us don't just toss random WIP and fixup commits on a branch and leave it to GitHub to squash at the end. I.e., it's the GitHub UI rather than Git that has been lacking.

(FWIW, I'm dealing with this sort of thing at work right now - working on a complex branch, rewriting history to keep it as a sequence of clean testable and reviewable commits, with a plan to split them out to individual PRs when I finish.)