Completely ignoring whatever butt about v20 vs v21, there would still be a serious community issue if it became common for people to use outdated versions of the game (Even in a best-case scenario where the master server handled versions in a way that allowed them to work individually for some reason).
People who host on different versions would alienate a large portion of the userbase from their content. It's seriously a terrible idea for people to start opting out of latest versions because the playerbase simply cannot withstand that behaviour. I don't think it's a problem for people to have the occasional private v20 server, but there's absolutely no harmless way for that to be commonplace. Minecraft can take it because Minecraft does not have a "community", and people always join by specifics anyway.