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.

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