Helldivers 2 moved from GameGuard to Easy Anti-Cheat on October 20 with update 7.2.0, Devoid of Liberty, and this is one anti-cheat change that doesn't lock out Steam Deck players. In its FAQ, Arrowhead said "Steam Deck and Linux compatibility remains unchanged." GamingOnLinux reads that as Arrowhead having enabled Proton support for the game.
GamingOnLinux says the update rolled out as it was writing up the change, and it confirmed the game kept working without issues on its Fedora KDE system. Its conclusion is that the game is still safe to play on SteamOS and Linux, including Steam Deck and Steam Machine. GamingOnLinux lists Helldivers 2 as Playable on Steam Deck.
Arrowhead says the switch is part of a wider push against cheating. Alongside Easy Anti-Cheat, it's adding extra security measures aimed at cheat mods that have been circulating recently. Mods that affect other players' games may stop working, and some harmless cosmetic mods may need updates from their creators before they work again. The studio said it appreciates cosmetic and non-gameplay mods, but it has warned before that it can't guarantee any mod is safe or usable.
Separately, Helldivers 2 and Horizon Forbidden West are still unavailable on Steam in the UK. GamingOnLinux says that goes back to a Steam back-end problem in mid-September that Sony is aware of but still hasn't fixed.