Has an MLB game ever ended 20-5?
Yes. MLB has seen a final score of 20-5 22 times. It first happened on June 14, 1876, when Philadelphia Athletics beat Cincinnati Reds, and most recently on May 26, 2022, when Cincinnati Reds beat Chicago Cubs.
That makes 20-5 the 171st most common final score out of 330 that have ever occurred in MLB. One point away, 21-5 has happened 15 times and 20-6 has happened 26 times. At one point the score disappeared for 50 years, between 1949 and 1999.