The Iron Judgement card back was originally part of the IJ expansion pre-order packs.
Some time after that it appeared as a separate item you could buy. I think after that it's been appearing and disappearing every so often. I'm sure both this card back and the other limited card backs will be back soon enough.
As for artificial scarcity; it's hardly limited to f2p games or games in general. People have always cared more about items they thought would soon be unavailable. So i'm guessing that you could probably trace tricks of the trade trade like that all the way back to the middle ages (likely earlier than that too but there's little chance evidence of that would still be around after so much time)