Has an NBA game ever ended 89-80?
Yes. NBA has seen a final score of 89-80 38 times. It first happened on February 26, 1948, when Philadelphia Warriors beat Chicago Stags, and most recently on January 18, 2018, when Philadelphia 76ers beat Boston Celtics.
That makes 89-80 the 633rd most common final score out of 3,189 that have ever occurred in NBA. One point away, 90-80 has happened 40 times and 89-81 has happened 46 times. Nearly all of its occurrences fall between 1980 and 2009.