Generally the advice is that public functions should have type annotations, and private functions needn't if you don't want to. But I could see arguments for either actually. Sometimes it's a bit of an annoying overhead if it's obvious what the function returns, and it certainly helps make prototyping code feel nice, especially since everything's typesafe anyway.

As I said in another comment, as the compiler knows the type of everything, it would be easy to have the LSP overlay the type (or annotate it into the text via code action for you).