Genuinely curious about this as well. It's a major bummer that self-hosted infra can't be used to validate GitHub Pull Requests now; basically means I'll have to move my entire workflow off of GitHub so that everything can be integrated reasonably again.
That's not exactly true. You just won't be able to use self-hosted infra to validate GitHub PRs a) using GHA and b) for free.
GitHub still supports e.g. PR checks that originate from other systems. We had PR checks before GHA and it's easy enough to go back to that. Jenkins has some stuff built in or you can make some simple API calls.
We use a combination of GHA and Jenkins jobs. All these end up as checks on GitHub. You could then proceed to say "Allow this PR Merge button if check A is ok and check B is ok" where check A arrived from GHA and check B from a Jenkins job.
