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