I hope nobody is counting javascript as a functional programming language. You can however write pretty decent functional code in C#.

I ported a 10kloc F# codebase to a very alien-looking functional-style C#. The project was processing several hundred thousand dollars woth of transactions per month and the go-live went flawlessly. I have since left the employer but am aware the code is still in production two years later and the C# devs over there almost never had to reach out for my help.

The long list of limitations C# has over F# is lead by: lack of first-class algebraic datatypes.