Support for Steam Deck != support for Linux version. Steam Deck use Proton to run Windows game on linux seamlessly.
Their direct competitor, Apex Legend, is steam deck verified. Big games like Monster Hunter World/Rise, Cyberpunk, Baldurs Gate 3, Elden Ring, etc etc, all steam deck verified. Check out this page for more info
It’s not a Linux problem, it’s a Tim Sweeny problem.