Do most libraries use static types? Or have something widely-used and well-supported available like the @types project for TypeScript to provide them, fairly seamlessly, to consumers of those libraries?
Some projects do, yes. Django, for example, has a library to provide types. In my experience, types are useful even just in my own application, since the libraries I use tend to generally only accept simple/fundamental types anyway. Things could definitely be better when it comes to library type support, though, I agree.