Well you have the problem of everything official not mentioning a capsule for the X-Wing. The B-Wing and some Y-Wings do do that apparently. In all the X-Wing books the seats just launch like a real world fighter jet. Then a shield pops on, either on the pilot or on their seat. Some comics have shown some kind of oxygen mask, but I'm guessing that would be manual where it's stowed on the seat and the pilot has to equip it. Basically it looked like the early SW stuff where they thought about using an oxygen mask.