Functional style in my opinion is about creating functions that operate on standardized data structures. Class based type systems actively discourage functional style, even when the language gives you a functional veneer over your OO code.

Haskell avoids this problem by using algebraic data types instead of class style types. Clojure avoids this problem by using dynamic typing.

I definitely do not associate 'functional' with static typing.

I've never been comfortable doing big projects - multiple files, more than a few hundred lines of code - in a dynamically typed language. It's the same reason were seeing the big uptake of typescript. Statically typed languages give us so many superpowers when we need to do refactoring or any changes at all to an existing codebase. Just changing a function name in a dynamically typed language can be hell. Rearranging the parameter order is even worse, cause you have such trouble finding all of the function uses and where you need to update. With statically typed languages you get editor support for simple stuff like this and it becomes automatic and a push of a button almost.