Yes, the official league (and playoff) version is first OOB, so that players coming to the league are not forced to learn and adapt to BM3.
I’m glad you asked, because I think I said the wrong thing before about the beginning of the year thing - I’ll need to edit that. EDIT MODE!
OOB will still be the official default in 2021 because the physical 1940 game that is widely available to the public is the most popular A&A physical game that is available for purchase by the public. So technically, a player could insist on the OOB version in the playoff, however, if that same player had never actually played anyone OOB during the regular season, then it wouldn’t be right (and would not be allowed) to spring it on an opponent to try and gain an advantage. The reason OOB 1940 is default is to not force new players to the league to change to a different ruleset, or in the case of path to victory, a whole new map. Practically speaking though, every year everyone generally plays the latest and greatest mod of the game that almost everyone in the league has been playing.
So I will not allow an unsportsmanlike “surprise” demand on an opponent to play a version that’s foreign to him. But if a player had been playing OOB (the default) in the league at least a couple or few games during the year, and insisted on OOB in the playoffs, that would have to be honored.