Should be the ability to freely remap their controls using the Steam Input interface. They've always worked on Deck as regular XInput devices.

There was a post from Valve a while back, you can now remap the back pedals via Steam, so they're now recognized as a different input button instead of having to use their software to mapping it to one of the existing buttons. At least that's my understanding, I don't own any 8bitdo, and this was the reason I never bought one, so that might change in the near future.