There are two cases we've seen recently where provisional PRs are left around indefinitely:

1. When a CL is abandoned
   (example: https://github.com/w3c/web-platform-tests/pull/6432)
2. When a CL no longer has exportable changes
   (example: https://github.com/w3c/web-platform-tests/pull/6288)

We want to be able to detect these and clean them up. #2 is tricky because a CL could go through the states (exportable changes -> no exportable changes -> exportable changes) so we'd need to be careful not to thrash the PR repeatedly opening and closing it.