I know this sounds against the jam but can we do it so that the obsolete feature of the game (such as a game mechanic) seems obsolete and pointless at first, but as the player learns more of the mechanics of the game, the mechanic actually becomes an interesting part of the gameplay loop that isn't shown as one?
Like for instance (and this is a mechanic I would totally want to see happen for a game someday) there's a feature that changes the aspect ratio of the screen for a platformer game. Which would seem ridiculous, but what if this aspect ratio can be used to get the player character stuck "outside" the game world and exploring a whole different level? Then they can come and glitch back into the game through the aspect ratio change. And so the world inside the game is a linear platformer, but the world outside the game is like a metroidvania in which you can push the game screen into new areas of the world and use it to solve puzzles.
The biggest part of this mechanic would be that nobody would be directly told that you can do this! People can completely look over the mechanic and just think the game is a regular old platformer, but the "obsolete mechanic" actually unlocks a huge part of the game! I know it goes against the whole point of the jam, and if the idea doesn't fit the jam's rules, that's totally fine (I'd probably just make that idea on my own time because I still want to see a game like that), I just wanted to make sure if that's cool with you and this jam!
Thanks!