Let me amend my previous statement, because I might have gotten it wrong.
It goes like this:
Say you’re really looking forward to the magmasmith. They’ve mentioned it is to be added, as they’ve also done multiplayer. But, magmasmith already has a folder in the games code, with the basics for the class. It’s 100% not allowed to create your own mod which “turns on” the magmasmith.
Multiplayer is the grey area.
Also, looking back, I do believe back when water wasn’t a thing, somebody made a water mod which “turned it on”, and that ended up getting shut down after a bit.
Oh. Also found the post where Brad laid down the rule.