Squashing is fine if you’re just making a mess of temporary commits as you work and you don’t want to keep any of those changes separate in master, but that’s not a useful review workflow. A lot of times I’ve built a feature in a way that decomposed naturally into e.g. two commits: one to do a preparatory refactor (which might have a lot of noisy and repetitive changes, like changing a function signature) and another to actually change the behavior. You want those changes to be separate because it makes the changes easier to review; the reviewer quickly skims the first commit, observes that it’s a mechanical refactor, and the change in behavior has its own, smaller commit without all the noise.

“What if there’s feedback and you need to make changes after the code review?” Then I do the same thing I did before I posted the code review: make separate “fixup” commits and do an interactive rebase to squash them into my commits. (And yes, I do validate that the intermediate commits build cleanly.)

There’s nothing you get from stacked PR’s that you don’t also get from saying “please review my feature branch commit by commit”.