Firebase Hosting custom domains API returning 501 errors

This incident has been resolved.

The fix is being rolled out. The ETA for complete resolution is July 31 at ~11:00 UTC, though some users may have restored functionality before then.

We believe to have found the root cause and are working to restore this portion of our API. We are more confident now that the data plane is unaffected and that this was limited in scope to the management of Hosting custom domains.

The engineering team is investigating elevated 501 "unimplemented" errors seen in Hosting's API when managing custom domains. It does not appear at this time that the data plane is effected.