Buttons, selector-based Notifications (though the block version is finally good, so we mostly just use that now), & gesture recognizers look like our biggest users of @objc. Basically all to support target/action.
A quick search through one of my apps shows that about 4% of functions are marked as @objc (and we're not using the old compatibility mode where more methods were implicitly @objc either).
@IBAction can replace @ObjC for all selectors. We put it a mandate in our code reviews. There is also NSObjectProtocol which forces (in most cases) devs to subclass NSObject.
A quick search through one of my apps shows that about 4% of functions are marked as @objc (and we're not using the old compatibility mode where more methods were implicitly @objc either).