What happens if an airliner emergency door is opened while the plane is in mid-air?

I believe the essential step was to depressurize the aircraft. And this should work with modern aircraft as well.

That would be necessary on a modern plane, where the doors initially open inward, but the 727 had a simple drop down rear door. Following the DB cooper incident, 727s were fitted with Cooper Vanes to stop the same thing happening again.

Boeing 747s had a few incidents with this type of thing too. See Pan Am 125 (no deaths) and United 811 (nine deaths.)

The examples of the United and Turkish Airlines flights would seem to indicate that survivability depends quite a bit on where the debris from the decompression goes. If it hits an engine or severs a line for a control surface, that’s problematic.