* kube-apiserver: requests to endpoints handled by unavailable extension API servers (as indicated by an `Available` condition of `false` in the registered APIService) now return `503` errors instead of `404` errors. ([#58070](https://github.com/kubernetes/kubernetes/pull/58070), [@weekface](https://github.com/weekface))

- Resolves a potential issue with GC and NS controllers which may delete objects after getting a 404 response from the server during its startup. This PR ensures that requests to aggregated APIs will get 503, not 404 while the APIServiceRegistrationController hasn't finished its job. ([#104748](https://github.com/kubernetes/kubernetes/pull/104748), [@p0lyn0mial](https://github.com/p0lyn0mial))