It is not a recently new feature. It has been in the game for a while.
We used to get some untradeable objects from TH in the past, but at a certain point Jagex made untradeable skins and change the tradeable objects to stackable tokens. For the token form of the prizes, the first one you receive is automatically converted to untradeable skin accessible from your customization tab. The next token(s) you get will come as item(s) in your inventory and you can trade them like all the other tradeable items.
25-Dec-2019 17:49:39