An airstair is literally just any built-in stairway on an aircraft—it doesn't have to be at the back. They continue to exist as options on the 737 and A320 families to this day, and are disproportionately specified by low-cost carriers (Ryanair have a lot of their 737s with them, for example), as although they're a weight penalty they decrease ground equipment required to service the aircraft (reducing costs), and can be deployed almost as quickly as the door can be opened helping reducing turnaround times (increasing sectors flown per day).