I don't know if there's a name for it but essentially F# is where the language designers can push the boundaries and try extremely new things that 99% of users will not want or need, but eventually some of them are such good ideas that they feed back into C#.

Maybe that's just research, and I'm glad that Microsoft hasn't killed F# (I do work there, but I don't write F# at work.)

> F# is where the language designers can push the boundaries

It really isn't, not anymore. F# now evolves conservatively, just trying to remove warts and keep up with C# interop.

And even then some C# features were considered too complex/powerful to implement (e.g. variance, scoped refs) or implemented in weaker, incompatible ways when C#'s design is considered messy (e.g. F#'s non-nullable constraints disallow value-types, which breaks for some generic methods written in C#, sadly even part of the System libs).