That feels like the opposite of what I think stacked PRs are? Like someone will open PR #1 for one feature, and then PR #2 into the PR #1 branch, but it doesn't make sense without knowing the context of PR #1 so that gets reviewed first - and then when that PR gets merged, the second one gets automatically closed by GitHub?

PR#1: dough PR#2: toppings

You first send PR#1, then PR#2 on top of the first one.

The diff for PR#1 will show dough stuff. The diff for PR#2 will show toppings in relation to dough.

People can review them asynchronously. If you merge PR#1, PR#2 will automatically target main (that's where dough went) now.

In this arrangement, I use to cross-mention the PRs by number (a link will exist in both). I also like to keep the second one draft, but that depends on the team practices.

I don't understand why you would close the second PR when the first gets merged. It should lose the dependency automagically, which is exactly what happens if you branch correctly.