I'd be surprised if we weren't allowed to use, say, the DSP-1 or the Super FX (if anyone's brave enough to try that one), but what about, say, the MSU-1? It's a chip made in the 2010s that allows the SNES to stream CD-quality audio and access up to 4GB of storage, which would make any game using it completely anachronistic, and arguably against the spirit of the jam.

Thanks for the question. I've just added it to the rules, essentially only expansion chips that were used in an era SNES game are allowed. So, like you said, something like SuperFX is allowed but MSU-1 or weird hacks like adding a modern microprocessor to your game are not.