Rust and Go pull in some stuff from functional programming (first class functions) but are not really designed around functional programming as the way to get things done.
Most go programming I've seen is like python where you are slinging around maps that are getting modified all over the place. Very antithetical to functional programming.
You are confusing functional programming for pure functional programming. This is like confusing Smalltalk-style OOP with OOP in general and then complaining about C++.
Mutability and functionalness are orthogonal, although the immutable style lends itself well to functional programming and mutability is more kludgy in FP.
But given that both Rust and Go have first class closures, they qualify as a functional programming language. The definition of FP is that functions are data. Python, Rust, Go, etc all fit this definition. Modern c++ does as well.
There is no reason not to include these languages, not only they support functions as data, they also support higher order functions (functions that take callbacks accepting callbacks accepting callbacks, ad infinitum).
Most go programming I've seen is like python where you are slinging around maps that are getting modified all over the place. Very antithetical to functional programming.