2010? damned! I must have missed a lot of things then
I thought the first were McLaren with there octopus exhaust (which required a slit to be legal) and then 2012.
An idea (maybe a silly one) just came to me: could they use the same trick to have DDD again? I don't know exactly how they ban them, I only remember there was an area where you could put holes to feed the other deck. So would a long slit from the edge of the floor to the feeding hole work, at least from a legal PoV (a slit all across the floor is not a thing you normally want)?