Oddly, the Hubitat OAuth servers are returning an error indicating:
"Hub is not responding, please verify it is running and connected to the Internet"
But the hubs are connected to the internet, don't have any connectivity issues indicated on the hubs themselves, and cloud endpoints continue to work as expected. For example, you can access Hubitat Dashboards using Cloud Endpoints or the Hubitat Maker API using Cloud Endpoints.
So it seems to be something limited specifically to the OAuth process rather than the Hubitat Cloud Relay or broader cloud connectivity from the hub itself.