OBD-II was mandated to be in new cars sold starting 1996, so while many cars prior to 1996 had sophisticated and sometimes OBD-II "semi"compliant systems, OBD-II didn't functionally exist before 1996 in cars sold in America.

BTW... i wonder if the federal law regarding OBD-II requires that all cars be computerized?

What if some company decided to make a 100% mechanical brand new car?