It has been. It's been server-side for decades. It's common industry knowledge that the client can't have authority. But server-side anti cheat can't stop aimbots or wall hacks. Client side anti cheat isn't about stopping you from issuing "teleport me to here plz server" commands, it's about stopping people from reading and writing the game's memory/address space.

If you wanted to teleport (and the server was poorly implemented enough to let you) you could just intercept your network packets and add a "teleport plz" message. Real cheats in the wild used to work this way. However a wallhack will need to read the game's memory to know where players are.

What modern anti cheat software does is make it difficult for casual cheats to read/write the game's memory, and force more sophisticated cheats down detectable exploit paths. It's impossible to prevent someone from reading the memory on untrusted hardware, but you can make it difficult and detectable so you can minimize the number of cheaters and maximize the number you detect and ban.

Linux is incompatible with client anti-cheat because there is no security boundary that can't be sidestepped with a custom compiled kernel. Windows is Windows, with known APIs and ways to read process memory that can be monitored. Secure boot means only Microsoft's own built kernels can boot and you now have a meaningful security boundary. Monitor what kernel drivers are loaded and you can make it harder for cheaters to find ways in. Sure you can run in a VM, but you can also detect when it happens.

Sure we can just run with no client side anticheat at all (functionally what Linux always is unless you only run approved, signed kernels and distros with secure boot) but wallhacks and aimbots become trivial to implement. These can only really be detected server side with statistical analysis. I hope you don't ban too many innocent people trying to find all the cheaters that way.

Honest question: given all the companies and people working on anti-cheat systems for the last 20+ years of multiplayer video games, don't you think it would all be server-side if it could be, by now?

Agree 100%, client side anti cheat was never going to work.