On the other hand, it's not a very advanced type level feature either to insist on total functions. I'm sure Rust's type system goes a lot further in enforcing a good level of type soundness.
No, it is basic. If you have a sum type, and you have no defined behaviour for one of the terms of the sum type, your code is not type-safe and a language that aspires to "basic type safety stuff" should at least issue some kind of error about it. In a putatively statically-typed language (ie one that does type checking at compile time), you'd expect that error to happen at compile time.
JS has this too in TS, so I’m pretty sure the perception among users of typed languages is that this is a basic part of what type systems do. What does this have to do with FP?
Edit: typo