I get your point, but it kind of is AI related. The size of PRs since AI has made it much harder to review a diff. Stacking the PRs seems like a response to this problem.

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