Sending copies of fake character data isn't a thing because there eventually has to be a flag that tells the client to not render that character that the client hack could simply read.

It should be clear that servers already do not trust the client, they do many checks hence you don't see teleportation hacks in games like Counter strike or Valorant. There used to be cheats in the counter strike games like "nospread" where you could have 100% pixel perfect aiming but that was because the the client was trusted however now in most games with some randomness in bullet spray patterns the random seed is different between the client and server so something like "nospread" are no longer possible.

You might be stumbling upon "fog of war" that is not sending data to a client unless the enemy player is close to visible which is a thing. It's widely used and I'd say effective in MOBA/MMORPG/RTS games however in FPS games fog of war is many times more computationally expensive which matters at the scale of games these days. It has been a thing for a long time in counter strike with server plugins like "SMAC anti wall hack" or "server side occlusion culling" however the implementations sometimes have not been perfect and require significantly stronger servers. https://github.com/87andrewh/CornerCullingSourceEngine

Riot games also implements fog of war at scale in Valorant and has a blog post covering some of the issues they overcame. One thing you can see the gif at the end of the blog post, even though fog of war is effective it is only effective in reducing the effectiveness of wall hacks and wall hacks still provide a significant advantage. https://technology.riotgames.com/news/demolishing-wallhacks-...