Saber Interactive have updated the co-op zombie shooter World War Z, and now they've disabled being able to pick the Vulkan API on Steam Deck.
It seems some users were reporting crashing issues with the game, as noted on the Steam forum post. A user emailed into GamingOnLinux to note they noticed that just recently after this forum post that now they can no longer pick Vulkan. And after contacting Saber Interactive support they said it was "disabled to improve the stability of the game".
So somewhere along the lines either an update to the game, Proton or the Mesa graphics drivers introduced issues for the game. For now out of the box, Vulkan just can't be picked on Steam Deck. This also means you can't use the in-game FSR 2.2 either, because it's only available with Vulkan in World War Z. Well, until you disable the Steam Deck check that is. Like you can do with Skyrim, you can set this launch option in the game Properties to get it all back:
As for some people, it seems Vulkan is actually more stable.
This change only affects Steam Deck. Testing it myself on Desktop Linux and you can still pick Vulkan normally with the default being DirectX 11.
World War Z is available from: