That is correct Nick, most rocket engine exhausts are not varable geometry primarily form a cost and weight standpoint. There have been some nozzles that had a two stage geometry where an extended "skirt" that was on the outside of the primary nozzle could be lowered into place at the appropriate point during the booster's ascent. But the vast majority of nozzles are simply sized for some point between the booster's liftoff ambient pressure and the ambient pressure at cut-off. That point being the one that yields maximum overall performance.