SA was proved to be almost useless - which some conspiracists think is the "real" reason that is was switched off (I am not one of them!). If you had a very accurately surveyed point and a GPS on it, you could tell what the error was and apply that to your (second) GPS reading of wherever you were. Not so useful a method for aviation, but for artillery or similar, SA proved to be a pain, rather than a hinderance.
SA was always on before it was abandoned, but they could vary the amount of error.