But as a player experience? It killed the game.
This was harder to spot on a demo recording because it didn't look like obvious aimbotting. It just looked like the player had "amazing game sense." Most CoD2 veterans will tell you: the worst players were the "togglers."
Imagine storming the beach in Harbor . You know exactly which doorway the sniper is hiding behind because you see a bright orange stick figure crouching there. You pre-fire. You win. The victim types "WTF?" in chat. A more sophisticated version of the wallhack wasn't visual. It was a radar overlay . The hacker's screen looked normal, but a small transparent map in the corner showed red dots (enemies) moving in real-time relative to the player's position.
However, modern private servers use heavily modified clients that encrypt wall data. But on vanilla GameSpy-replacement servers?