When we have a new version come along that becomes very popular, we have to address what version will be played in the playoffs. As in the past, I think the sound policy is to make the version that was most widely accepted at the beginning of the league year (1/1/20) the default version in the playoffs. That is, a totally different version may still be played in any playoff game if both players agree, but one player absolutely has the right to refuse, and demand the default version, no questions asked. That way everyone can count on being guaranteed to be able to play the default, and not some new-fangled version they’ve never seen.
Edited - I mis-spoke there. The technical default has been and will continue to be OOB 1940 because it’s the base-line game that is common to everyone including newcomers to our league. In practice, the vast majority of playoff games are played in the most popular mod of the game during the year, but if a playoff contender is most comfortable with OOB 1940 he can insist on it in the playoffs. However, if that same player had never played a single OOB 1940 game in league play that year and still completed 8 games and qualified for the playoff, I would not let him insist on playing OOB against somebody because clearly he is comfortable with mods, having qualified with 8 games not playing OOB at all.