Works fine on non-Windows systems (well, Linux and macOS at least) IME and isn't several years behind.

Also I think you are giving nullable ref types too much credit

Having the type system be able to tell you that you may have a null reference exception in a complicated code base is magic. I've inherited bad code bases and refactored them to use nullable references and watched lots of bugs fall out.

That said, I get where you are coming from - .net framework is pretty much just windows, and there a large number of libraries/apps that are Windows first, other platforms second.

If you do end up needing to do Windows dev, though, I'd recommend it, same as if I were doing Apple Dev I'd recommend Objective C/Swift, or recommend you brush up on bash/zsh/fish, C and maybe Xorg/Systemd for linux.

You can use other languages on all these platforms but the 'native' toolkit tends to enjoy better interop and have better support.

Edit: The other reason Unity developers are careful around new language features is avoiding memory allocations. When you write C# like it was Java it's more straightforward to see where allocations happen but it's all too easy to accidentally write allocations in your main loop if you use everything the language has to offer.