Not sure about the old days, but at the moment FAA minimum crew are required to board passengers, though one FA can leave to perform safety duties related to the flight in question only and must remain in the immediate vicinity of the boarding door. For deplaning, as well as when at an intermediate stop when no boarding is occurring, up to half the required FAs can deplane while passengers are still on the aircraft. In all other situations, when passengers are onboard, FAA minimum crew must also be on board.