Earlier quoted context omitted.
I'm a little skeptical that that's the case. When I cheated in CS:GO years ago (1856 days ago in 2016), I specifically remember being unable to see where people went at the beginning of the round, and sniping double-doors on dust2 showed people only when they were about to pass by the opening. This was on official competitive servers, so maybe you only mean self-hosted servers don't have FOW.
Another source engine cheat dev here. Source has had dormant entities at long ranges in culled-off areas [sectors of the map demarcated by func_areaportal] since... as long as I can remember. Your cheat couldn't see those players because the server was doing a very cheap and primitive visibility calculation, deciding the player entities are dormant for you, and choosing to not send you the data for them. This is diff…
Would an anticheat work if you did the opposite of this? Send the client random movement data above and behind them and if they spin and shoot towards it instant permaban forever
Random wonder. I have no idea in or around anticheating, please do feel free to shred it as an idea to pieces