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.

Meh. Stacked PRs (where a PR is already a stack of commits, and reviewers can perhaps communicate with each other to coordinate scope/responsibility) are a bit like tab groups in the browser when tabs, multiple windows, and workspaces already existed for maintaining organization. This new feature clearly scratches an itch for some users but creates more complexity (yes, even if you don't interact with it) for people already comfortable with the status quo.

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

Not AI related, but I plan on having my agents use this heavily so I can review PRs in bite-sized chunks instead of all at once.

This is definitely connected to AI. High code volume, bigger PRs.