Discussion among members of the development team.
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For multiplayer, that would have to be based on the other players' versions too. If the UMC author wants both the old and new versions of their multiplayer add-on to be available, I think they should publish the two versions as separate add-ons. That way it's easy for all players to join a host with the old version, or for a host with the new version to rehost if a player with only the old version requests before the game starts.
I don't think it's a straight up bad idea to add new features to stable, but I would say you should probably only add small features. Supporting a new type of widget in the Lua dialog system, or anything that's pure Lua in implementation, are probably find. Major new features are definitely not okay. And of course anything that changes syntax or keys should probably be avoided, but if added must have a compatibility path so the old syntax continues to work.