The syntax to declare types is specified. What to do with the declared types is then to be specified by a specific implementation.

Typical possibilities:

* ignore

* use for optimizations

* use for type assertions and type checking

It is. SBCL does a lot of really nice things and it would be nice if someone who were to design a modern Lisp would take inspiration from it.

That said, Common Lisp is hyper advanced alien technology, so it is hard to improve upon.