It seems like url("register/confirm/{$...e->confirmation_code}") targeting url() can also be of type object<Illuminate\Contracts\Routing\UrlGenerator>; however, Illuminate\Notifications...SimpleMessage::action() does only seem to accept string, maybe add an additional type check?
This check looks at variables that are passed out again to other methods.
If the outgoing method call has stricter type requirements than the method itself, an issue is raised.
This check looks from parameters that have been defined for a function or method, but which are not used in the method body.