Autopilots are considered to be capable of up to 4 degrees Glideslopes after that you're in "Steep Approach" territory. The appropriate rules are NPA-25D and the critical requirement is that the aircraft must be able to fly down a slope 2 degrees steeper than the desired approach without the engines hitting idle - no mean feat. The same autopilot control laws are used by aircraft both capable and not capable of carrying out steep approaches - it just comes down to aerodynamics.