A Valve spokesperson has confirmed that Steam's Big Picture mode will be replaced by the Steam Deck's user interface. Big Picture mode is Steam's version of a console-like user interface: It can be navigated using a controller, and was pretty slick back when Steam Machines were still possibly going to change the console landscape (spoiler: they didn't). To say that Big Picture has grown long in the tooth is maybe an understatement, as it hasn't received a major refresh since 2015.
Queried on whether the Steam Deck UI will replace Big Picture in the Steam forums, a Valve moderator, austinp_valve, answered in the affirmative. "Yes, we are replacing Big Picture with the new UI from Deck," reads the message. "We don't have an ETA to share yet though."
Based on the small glimpses provided already, the Steam Deck UI looks unsurprisingly superior to Big Picture mode. Once rolled out, it means that if you want a gaming PC to act like a console, but either don't want to splurge on a Steam Deck or don't care for portable gaming, you can do that pretty easily.
Big Picture mode launched into beta in 2012. If you want a glimpse of how the mode looked back in the innocent days of 2012, this thorough walkthrough is quite a trip. Steam Deck will start shipping in December, if you were lucky enough to get a reservation.