Camera+ didn't use a hidden API - they just used a standard system volume control but placed it offscreen and monitored if the volume changed (and then lower it imperceptibly if you hit 100%). And Apple has still not added an API to do this officially, last I checked all third party camera apps use a similar hack.

What changed was just the interpretation of the app store rules - the rules said something like "you're not allowed to use hardware buttons in a non-standard way". Once they added the volume shutter feature to the system camera, now it's not a non-standard way and it's not against the rules.