Steam Deck

To the best of my knowledge, no kernel level anticheat systems support Linux yet. So it's not something you have to worry about unless you're on windows.

Kernel anticheat is considered bad because it has full access to your computer. It's able to see everything happening on the computer to make sure you don't have cheats running. This is obviously bad for privacy reasons, and this is made worse because most games that use kernel anticheat are Chinese owned. The Chinese government is well known for spying on their citizens, and it's a very reasonable assumption that any kernel anticheat from a Chinese company is required to have backdoor access available to their government.

Other concerns are that it lowers the safety of computers. Kernel level access makes it very easy to compromise the computer. Genshin impact used to use kernel anticheat, but the anticheat was used to infect people with ransomware.

Furthermore, there's an ever increasing number of cheat options that don't run on the computer running the game, meaning that kernel anticheat can't even prevent cheating in games. An example is Aimbot software that uses a stream card to get screen info, and sends mouse inputs to instantly get headshots.

afaik EAC isn't kernel-level so it could be much worse

No, and it's been used in lots of games for years. Enigma Protector, the one that Capcom is retroactively adding to their entire library of games on top of solutions like EAC, comes from a shady Russian company and has been breaking games. That's why you're seeing talks about kernel-level DRM, lately.

ETA: Enigma Protector is kernel-level and blocks cheat engines, enforces DRM, and most importantly, prevents anyone from modding their games. It's yet another example of an insane Conservative pushing purity culture ideals onto everyone under the auspices of capitalism.

What do you mean by "harmful"? As far as I'm aware, kernel-based anti-cheats like Valorant's don't interact with the hardware, they only check that Windows hasn't been tempered with.

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.