Has an MLB game ever ended 18-9?
It has. 18-9 has been a MLB final 28 times in 235,971 games. The first was Boston Red Caps beat Chicago White Stockings on May 12, 1877; the latest was Los Angeles Dodgers beat Cincinnati Reds on August 22, 2016.
That makes 18-9 the 159th most common final score out of 330 that have ever occurred in MLB. One point away, 19-9 has happened 16 times and 18-10 has happened 15 times.