It's pretty simple: they've always had life support.
Keep in mind that the only reason X-Wing pilots aren't seen wearing breathing apparatus on their helmets in ANH as originally intended is because Lucas felt it important to be able to see the pilot's faces. There's even some photos of the prototype floating around online, which clearly still has the breathing apparatus attached.
In-universe: remember that the Empire is big on suppressing outward individuality. Do Stormtroopers really need to be wearing their helmets (which can also be vacuum sealed) all the time, even while on space stations and starships where there should be no immediate environmental hazards? Of course not, but it's all part of the ethos and mentality of the Imperial military. Recall how Phasma chastised Finn for removing his helmet "without permission" after they returned to the 'Finalizer'? It was probably no different back in the old days of the Empire.
In the case of the pilots it has the added bonus of being practical that in the event of a breach, they're already suited up, they just need to activate their seals and turn on their closed oxygen supply. Possible downside: restricted vision and an over-dependence on heads-up telemetry perhaps?