Was the 727 the last airliner whose doors could be opened in flight?
The door of an airplane can be opened in any of the aircraft only when the airplane is depressurized. When an airplane is pressurized, the extra pressure inside the hull of the plane forces the door outwards which in turn presses on a seal and hence, to open a door in such a configuration, where 6 p.s.i. (pounds per square inch) of pressure is acting against several square feet of the door, one would need a tremendous amount of force, and even if the door was to be opened electronically or hydraulically, one would still expect rapid decompression of the cabin after opening the door.

Hence it is categorically incorrect to claim that modern (new) airplanes can't open their doors mid flight. It is quite achievable, but after depressurizing the cabin. Here is a source for your further reading. 

Edit

The following airplanes have Air-Stair installed. I'm not sure which of them has an over-ride to open this Air-Stair mid-air, but any airplane which does not have a cooper vane installed (Or any other external barrier) can open the Air-Stair midair (technically):

Lockheed L1011

Boeing VC 25

Ilyushin Il-86

DC-9/MD-80/MD-90/B-717

Boeing 737

Some modified Airbus and Boeing planes have Air-Stairs too.