> So frustrating that Github doesn't support such an obvious workflow.

It kind of does.

I used this a lot in several jobs to work in dependent tickets in advance. Just make another branch on top of the previous (a PR to the other PR branch).

People could review the child PR before the parent was merged. And it requires some less than trivial git knowledge to manage effectively, but nothing extraordinary. Any solution for stacked PRs based on git would also require it (or a custom tool).

I think I'm on their side on this one. From git perspective, it works just as I expect. Something else probably belongs to JIRA or project management instead.