As long as rendering happens on the client, it's going to require client-side anti-cheat to stop players from looking through walls or adding aimbots.
Client side rendering means the client has information like the location of the opponent even when the opponent is not visible. The server is unable to tell on its own whether the client is using this information inappropriately.
Other techniques include simple statistical analysis: it's usually pretty damn obvious when a player is making inputs that shouldn't be humanly possible consistently, or reacting to information they shouldn't be able to see. That doesn't have to be done in realtime, it can be stored and reviewed later. (That might even be a feature on its own: delayed bans make it harder for cheaters to know if their techniques were detectable.)
Bans have typically been done in waves for a long time for exactly this reason, yes.
So if you're sending that data to the client, well now cheats can look for that and detect if there's a player behind you and warn you, giving an advantage.
The same thing applies to corners and other occlusions, just less extreme, but still just as frustrating.