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
Switching back and forth between stable and experimental versions is not a counter argument to native games being superior.