Has an NBA game ever ended 90-80?
Yes. NBA has seen a final score of 90-80 40 times. It first happened on November 23, 1948, when Ft. Wayne Zollner Pistons beat Philadelphia Warriors, and most recently on December 21, 2018, when Chicago Bulls beat Orlando Magic.
That makes 90-80 the 614th most common final score out of 3,189 that have ever occurred in NBA. One point away, 91-80 has happened 41 times and 90-81 has happened 30 times. Nearly all of its occurrences fall between 1980 and 2009.