It switched from the standalone version of EAC to Epic Online Services anticheat instead, breaking steam deck support in the process.
However, EOS is normally supported on Steam Deck, so this is probably a temporary issue and will hopefully be resolved by either Valve or Respawn/Epic. We’ll just have to wait and see.
Edit: this is reportedly now fixed on proton experimental
I know it was a pretty early one to support the deck, but games like CS;GO supported Linux for many years before the proton was a thing.