When your App Service is upgraded to a new technology, administrators should get a notification, and the change should be logged. For example, when the upgrade from .NET Framework 4.7.1 to 4.7.2 occured.

We had an issue recently where several of our clients were using a CMS that completely broke on .NET 4.7.2. Had there been a notification or a log, we would have immediately seen that .NET was upgraded and then the site went down. Instead, we spent hours troubleshooting a known issue. I understand why Azure can't give every client a maintenance window for minor changes like this, but logging it when it happens seems prudent (and notifying admins with an email, even better).