Correct, Cicotte won 21 in 1920 and then was banned for throwing the 1919 Series. Same deal with his teammate Lefty Williams, who won 22 that season.
The only other pitcher in the 20th century to accomplish "20 and done" is Henry Schmidt, who went 22-13 in 1903 for Brooklyn. He wrote the Superbas prior to the 1904 season that he would not be returning because, "I do not like living in the East and will not report." Schmidt was a 30 year old rookie with an ERA+ of 84, so I don't believe he was throwing away a Hall of Fame career, but still.
Another pitcher almost joined Mussina in his "20 and done" season. Brandon Webb had 22 wins in 2008; he played one game in 2009 (I assume he was injured in that game?) and then never returned. I'd forgotten how good Webb was in his final 3 seasons: a Cy Young award and two CY runner-ups. Finished at 29.