IIRC all 02+ Impreza's have the capability of it...it just needs to be installed. its behind the glove box assy, it has to come out in order to get to it. Its a PITA.
Only if requested, its an additional cost for the filter and labor. Labor varies by car, as some are very easy, the Legacy 2005+ is difficult, the Impreza is imbetween...also filter prices differ by car. it used to be a part of the 30k before the new system went in, but because most of the cars we service either dont have it, or have the capability but the customer doesnt want it...we took it off the list of things to do for the 30/60/90/120 services.