I think the way to go about detecting cheats server-side would be primarily driven by statistics. For example, to counter wallhacks one might track how often a player is already targeting an enemy before they become visible. Or to counter aimbots one could check for humanly impossible amounts of changes in the direction of mouse movement, somewhat similar to how the community found out a bunch of cheaters using slowmo in Trackmania.

Add in a reputation system that actually requires a good amount of playtime to be put into the highest tier of trust for matchmaking and I think one could have a pretty solid system that wouldn’t have to rely on client-side anticheat at all.