Just want to add that via some shenanigans below you can already get the extra bumpers/buttons to be mappable through steam (I believe this only works for Bluetooth mode at the moment)

https://redlib.astrial.org/r/SteamDeck/comments/1j3gfch/getting_the_8bitdo_ultimate_2_to_work_in_steam/

It’s a fantastic controller. The only flaw IMO is a lack of rubberized grip. I added a little grip tape and now I prefer it to my old Elite 2.