On the Steam Deck, while using SteamOS (or other Linux distros), EAC (and a few others like Battleye) run in userspace, not as kernel level.

The intention of Anti-Cheat and DRM is to hide what they’re doing, in an attempt to prevent people from cheating or pirating. Malware often uses similar techniques to hide what it’s doing.

Kernel level Anti-Cheat runs with the highest level of permission on your system, meaning it has access to everything happening on your PC, and all your hardware.

That means kernel level Anti-Cheat can do whatever it wants on your computer, and it’s intentionally hard to figure out what it’s doing. Even though it’s probably not harmful, it shares a lot of similarities with actual malware, and we can’t be fully sure whether it is harmful or not. This is why a lot of people are against kernel level Anti-Cheat.

EAC, afaik, has acted as just an anti cheat, and is therefore likely not harmful to your system. However, like other Anti-Cheats, it is harmful with the standards being set.