Update: Valve developer Pierre-Loup Griffais mentioned on Twitter it’s for the upcoming Star Wars Jedi Survivor.
Here's something interesting for you. Valve developers seem to be improving the Mesa graphics drivers right now towards a secret game. First noticed by Phoronix, developer Samuel Pitoiset has been working on improvements in Mesa for the RADV driver (AMD Vulkan). This is the driver used by most for gaming on AMD + desktop Linux and also the Steam Deck.
In a merge request from April 18th, one of the commits noted there was an improvement to "a depth-only pass by +35% in one upcoming game". Another one from April 19th notes it gives another performance improvement of around 10% in "that game" (continuing to be rather cryptic). Then from April 20th there's another and when you read into the comments, Pitoiset notes it's "again this upcoming (secret) game" that was pointed out by SadlyItsBradley on Twitter.
Initially, I thought perhaps it was just going to be preparations for Star Wars Jedi: Survivor, and they just didn't want to name the game of a partner they were testing for because it's coming soon. However, the continued secrecy around it and even saying directly it's a "secret" definitely now has my curiosity piqued.
Do we think this is a new Valve game? It would make sense that Valve have their people working on performance improvements, and not naming it because then the internet would go wild. Perhaps Left 4 Dead 3 that was spotted being named in Counter Strike 2 files? Perhaps finally Half-Life 3 (yeah likely not but I can dream).
Pure speculation of course and it may not be a Valve game at all. It really could actually just be a game they're testing for a third-party, as we know Valve do get to do driver fixes and Proton fixes at times before a game releases on Steam so it's ready for Steam Deck. Either way, it seems we'll eventually find out on this one…