For most hill assist systems, its mechanical, not electrical. MOST that is.
1. It DID come on some 07 models. My FXT had it. (I know, that isnt a WRX, but just saying the 07 MY, some had it)
2. It isn't something you can turn on and off. But it can be effectively disabled. (see first line)
3. Its MECHANICAL, not electrical. This is why you cant turn it off. There is no "off" You cant unplug it, you cant switch it off. You can adjust it so loose that it doesnt work any more.
4. It has nothing to do with your transmission. Its not part of the transmission. You can have it on a 6 speed. If you swap in a 6mt and its gone, you somehow disabled it (possibly just left it off). I know this for a fact, and as proof, well, I have a 6MT and, hill assist.
It isn't some thing you can change a setting. Its a VERY simple device. When you hit the brake pedal and the clutch, it engages a small valve, that holds the brake pressure in the lines until you release the clutch. This is normally fine and not a problem, but Subaru has a HUGE tendency to ship them from the factory with it adjusted so tightly that it engages on flat ground. If you adjust it properly, its seemless and damn near impossible to detect unless you are really looking for it. It will drop off as soon as you start to lift off the clutch even the tiniest bit on a hill, so your footwork is exactly the same as it would be without hill assist. The only difference is, when you are sitting still you don't have to hold your brake.
For those of you with electrical assist.
USUALLY you can disable it without kicking any lights on by adjusting it looser. Be it electronic or pure mechanical, the valve is still a valve, and almost always adjustable. If you just adjust it looser, it will be less, or loose enough and it just wont function. Lights don't come on until you unplug things typically.
Are you guys SURE its electronic on your cars? I highly doubt it. If so, regardless, find the adjustment and loosen it at the valve itself. Its usually near the firewall by the drivers side frame rail.