HTTP Response header say the server is running IIS 10.0 so its most likely Windows Server 2016.
https://docs.microsoft.com/en-us/aspnet/core/fundamentals/se...
https://docs.microsoft.com/en-us/aspnet/core/fundamentals/se...
You can also build your reverse proxy using Kestrel.
Though IIS is more battle tested so they may be using that as front line server for such a high profile service as Bing
Near future: Windows Server 2019 + http.sys + ASP.NET Core
Medium future: Windows Server 2019 + Kestrel + ASP.NET Core