Not discriminated unions, but they're coming (I think next version of C#). Although for now you can simulate them quite easily:

    public abstract record Either<L, R>;
    public sealed record Left<L, R>(L Value) : Either<L, R>;
    public sealed record Right<L, R>(R Value) : Either<L, R>;

Pattern-matching works well with these simulated algebraic data-types. Obviously, exhaustiveness checks can't work on 'open' types, so it's not perfect, but you can unpack values, apply predicate clauses, etc.

Other more niche features like type-providers don't exist either (although arguably those could be done with source-generators in C#). It's been a long time since I did any F#, so not sure if there's anything new in there I'm unaware of.