Has an MLB game ever ended 20-6?
It has. 20-6 has been a MLB final 26 times in 235,971 games. The first was St. Louis Brown Stockings beat Boston Red Caps on June 14, 1876; the latest was Minnesota Twins beat Cleveland Guardians on September 4, 2023.
That makes 20-6 the 163rd most common final score out of 330 that have ever occurred in MLB. One point away, 21-6 has happened 8 times and 20-7 has happened 16 times. At one point the score disappeared for 26 years, between 1917 and 1943.