Can I play the Steam Deck handheld (with controllers attached), on kickstand with controllers detracted and split, docked with controllers in a dock to make a single controller, and docked with controller split and turned sideways for two player?
Sort of, yes - at least, the linux kernel has built in support for Nintendo controller connections over bluetooth, and there are some userspace drivers (or dkms, or maybe they've been merged into mainline now, `joycond` iirc) for support for split/merged/2p joycons. There's the Steam Dock peripheral for playing it docked to a TV just like the switch. The only thing missing is the ability to stow the joycons into the body of the deck (it has a built in controller so you don't need the joycons to play handheld), and you'd have to get a case for the deck with a kickstand as there isn't one built in. I've been using a Switch Pro controller to play TotK via Yuzu and it works perfectly, gyro included.
Obviously you can't attach controllers directly to the deck, but it does support a ton of controllers including the Switch ones. With motion control etc.
