I think they have been removed from C#10 now, I saw this on the language discussion:
>While we wanted to look at discriminated unions for C# 10, we don't think we're going to be able to
F# has the advantage that the features are very well considered and come from a functional programming heritage. The fact it's a functional-first language rather than a bolt-on like C# is a massive advantage.
The functional features in C# are not like for like ports of F# features and have very large gaps in the functionality and usage. It's great that C# is looking at F# to try and stay relevant but it's massive dishonesty to claim those features are just the same as those in F#.
The basis and usage of a language are steeped in its construction and usage patterns, F# has a distinct base of applying pragmatic functional programming and simplicity, whereas C# is an object oriented language takings usage patterns from Java and the typical patterns formed from that area. They will never converge due to the huge amassed different historical usage patterns. You could say C# is heading for being the new Scala 2 when Scala 3 has learned the lessons of blindly applying features and has now taken a more reasoned approach.