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This Xbox service runs on Kubernetes: https://azure.microsoft.com/en-us/blog/building-xbox-game-st... (I work on Azure, focusing on Linux and open source technologies)

At least some of the Halo games used Orleans as part of the multiplayer backend:

https://github.com/dotnet/orleans

Orleans can run on .NET Core now, but I don't believe .NET Core existed when those backends were first created. So they would've been running under the full .NET Framework on Windows.

The keyword that seems to be missing from both of those links is "Windows". Seems they are running on Azure alright, but that they are using Windows doesn't seem to be so sure.