I notice in the source code there is already code for closed issues to be sorted on creation date instead of issue number. That seems odd, since those two will normally have the same ordering. Though it is descending instead of ascending. Still, I wonder if this was a thinko and it was really supposed to be using when an issue was closed.

It looks like for issues the close date isn't tracked. Ordering by the latest comment might be close enough and wouldn't need a schema change.
Ordering by the latest comment is something that some people might find useful for open issues and PRs, as it makes it easy to find what's new.
PRs do appear to track the close date with the closed_at field.

So the part for PR is actually already done: https://pagure.io/pagure/pull-requests?status=0

The part for ticket isn't but is covered in https://pagure.io/pagure/issue/773

Closing this bug as duplicate (well invalid since 2.2.1 doesn't support duplicate yet)

For PRs, the close date is there, but the ordering is ticket number rather than close date. I can propose PRs later for this, instead of keeping the issue open.