The subscription wasn’t to discourage people from modding, and $20 is still very cheap and would mean they could do a lot more than just mod your game. Even then, they wouldn’t have to have an active subscription to be able to do that, they would only need to do it once and get the version of the engine that your game uses.
Also, I would look at other games and consider how important mod support really is, there’s plenty of big games even that people don’t care about modding, smaller games have an even lower interest in that.