Updated my comment for clarity - yes, SQL Server and Windows for their .NET app. Maybe they've moved to Linux if they have finished their migration to .NET Core, but even if they did, it's a fairly recent thing.
They're using .NET Core, and I think even .NET 5 in the meantime though I couldn't find a clear confirmation for that. But they're still on Windows, looking at Linux and containers for potentially in the future.
As far as I know they have been using .NET Core (or just EF Core? would be weird to go EF Core + .NET Framework, I guess?) on prod in 2018:
>This is the query pattern that caused StackOverflow.com to go offline yesterday:
https://github.com/dotnet/efcore/issues/13524
So maybe they used some things on Linux and some things on Windows - those that couldnt be ported to .NET Core back in the days