On: Official proposal for Type Unions in C#

[neonsunset]

There is one: https://www.freshports.org/lang/dotnet

    pkg install lang/dotnet

Note that not everything will work, and it is community-supported. But the effort is there, and there are contributions to dotnet/runtime from time to time to further improve or fix it. It's in a better state than it was 2 years ago.