The A320 family was always offered with airstairs from the very beginning, but no airline customer has yet specified them (to my knowledge) Most recently easyJet were offered the airstairs option when Airbus were competing with Boeing for the initial A319 order, but easyJet decided against it - on the grounds that it adds a fair amount of weight, doesn't increase the speed of a turnaround, and since airstairs are steeper than standard ramp stairs there was a possible health and safety argument against their use for passenger boarding/disembarking.
British Airways very early A320s were built with the Airstair option, presumably a BCal requirement, but as far as I know they were delivered to BA
without airstairs. However you can still see the airstair door undeneath the L1 door on their CFM-powered A320s.
Air France A320-100s built at the same time do not have this option...
The only active airstairs I've ever seen on A320-family aircraft are on the A319CJ...
So in theory any future A318/319/320/321 can be built with integral airstairs if the customer requires it, but it's unlikely any commercail airline customers will specify it now due to the cost/weight and possible safety implications.