It's not, although Microsoft absolutely tries to make it much easier to write/deploy/maintain C# in an all-Azure tech stack. That's good or bad, depending on whether you have other reasons to be on Azure.

I'd say Visual Studio (not VSCode) is pretty much the only not-really-optional, not-really-free dependency. I suppose that means you also need to buy Windows.

Personally, while I use(d) C# for work and I think it comes with a solid toolchain and I have no big complaints, I have never chosen it when I could choose the stack from the beginning.