Steam’s Big Picture mode hasn’t had an update since 2015, and it’s not going to get any more, as it’s facing the axe. A question in the Steam forums, as noticed by PC Gamer, about whether the Steam Deck UI would replace Big Picture, was answered with a ‘yes’.
There’s slightly more to it, but not much: "Yes, we are replacing Big Picture with the new UI from Deck," reads the reply from a Valve moderator. "We don't have an ETA to share yet though." Big Picture is the default interface for using Steam via your television, Raspberry Pi or Steam Link device.
With a native resolution of 1280x800 pixels on its touchscreen, and the ability to output to an external display, the Steam Deck UI needs to work well whether being controlled with flicks of the finger like the world’s bulkiest cellphone or with a gamepad/mouse and keyboard like a console. Big Picture worked OK for the latter, but anyone who ever streamed Steam to an iPad will know it just wasn’t designed for touchscreen interaction and as such Big Picture would not have been a suitable option for the Steam Deck.