There's been an increase in games that don't give the client full knowledge of enemies. That data doesn't actually need to be sent to the client if you can do checks on the server to know if they're visible. Yeah, it needs to be simplified from a full raytraced solution from the camera, but it can be good enough that it isn't much of a issue, depending on the game.

IIRC, some game (it may be Counter Strike, but idk) only gives your client player data for the "room" you're in, and adjacent ones, or something like that. You can still see through walls near you, but you can't see people on the other side of the map.

Yes, there's always going to be a point where there's nothing more you can do and you just have to hope for the best, and mitigate what you can on the client. Still, the naive "the client has to know where the enemies are" isn't accurate. A well designed anti-cheat solution will try to come up with a solution for this. Sometimes it isn't possible, but often there's some amount of information that doesn't need to be sent to players that can be hidden.