I read the same info on 999#’s. I did some research quickly and it backed that statement up. The Universities used 999 for foreign students as an ID. SSA has never issued a number above 799. They say have plenty of numbers left to still issue without repeat.
999 SS#s make perfect sense to anybody who knows about older computer systems. Back in the day, programs frequently REQUIRED people to fill in a field with data, even if it didn’t apply. ALL 9’s was the common answer: It was the catch-all entry that served as filler when there was no actual NUMBER to enter. So it make sense that foreign students would have their database records filled with 9’s on SS#.