According to the "Chronology of Basketball Rules" in the Basketball Handbook, which is genuinely fascinating reading, the two-foot center circle was deleted in 1995. I assume that's the '95-96 season, but don't know for sure.
It was the same season that the "double bonus", the three-tenths of a second rule, airborne-defender exception to over and back, and disqualifying bench personnel who leave the bench area for a fight rules were added.