It's entirely possible the move to Azure has made the availability problems worse. Dedicated hardware is much more predictable than cloud. "Let's not move to Azure and instead buy a few more racks" was likely a decision beyond the pay grade of github's management.
I mean its Microsoft and its Azure. How much can go wrong clicking yourself a few/hundred non autoscaling normal VMs?
There is so much workload running on Azure, i never heard of VMs go away.
If Microsoft can source hardware for Azure, Microsoft can source hardware for Github.
Azure is easily the least reliable and least secure of the 3 hyperscalers, which is crazy because GCP was an also-ran underdog not that long ago.