We were benefiting from stacked pull requests in Phabricator for a decade before "AI" was a thing. Having well scoped commits that can be individually actioned by distinct sets of reviewers has always been extremely useful.

It definitely isn't. People have wanted this for many years. It's an obvious workflow. There's been a feature request since at least 2020 and I'm sure there were earlier ones: https://github.com/cli/cli/issues/2693