I'm one of the co-founders of UP promod, former admin at UA, DF, EP, IceOps, and also a long time player. I think it's fair to say I have quite some experience with catching cheaters.
You saying that you've been cheating on UP servers without getting caught is painful to hear. We have quite a lot of admins and some of them with a lot of experience, so we definitely don't rely only on screenshots. Actually, despite us taking screenshots regularly, we primarily take demos and we investigate the demos. We also spend a lot of time in spectate just like in the old days. A lot of our admins play in under-cover so that nobody knows whether there is an admin watching. Screenshots are a convenient rough filter, but you should never rely on screenshots only and all our admins know that.
When it comes to anticheats, I don't think there is any hope, to be honest. There are entire companies that focus solely on making anticheats and even though they have quite a big team of developers focusing on this one task, usually the anticheat is bypassed shortly after it's released. Take Warzone as an example - brand new anticheat system, yet the game is full of cheaters. And we don't even need to go that far... The new FPS-Challenge anticheat for cod4 had been bypassed few days after it was rolled out and I've heard that even some older cheats are still not detected either. So as it has already been pointed out by @Kreator, spectating players, taking demos and inspecting the demos is the only reliable way to catch cheaters today.
But I will also say that there are a lot of players who think everyone is cheating, when in fact the accused players are just really good at the game. The skill in cod4 has gone through the roof lately, especially with the rise of cod-wars, ladder and cups. And you can see that even the former pros can't always keep up with the current competition. There are players who have been playing the game for 13 years on more or less daily basis and they know every pixel of every map. They have their aim nailed down to the point that they don't even need to look at their screens to land a shot if they know where exactly the enemy is. This isn't cheating, this is skill that I've see on my own eyes.
On public servers, ghosting is also more common than ever. Screenshots can sometimes tell you (because of discord overlay) who is ghosting. This isn't something I'd ban for, but I usually warn these players or move them to the same team so they can no longer ghost on the others.
At UP, we have a player-reporting system and we get a lot of false alarms on daily basis. Unfortunately, there are still a lot of cheaters and we do everything we can to catch them, but the ratio between the reported players and real cheaters is quite high.
As it has already been explained here, there is no cheat that would give you higher damage, at least not one I'm aware of, because this information is processed on the server-side, not on the client. Modified mods may be the culprits here.
High-ping players killing lower-ping players is nothing out of the ordinary. CoD4 is known to be advantageous towards high-ping players than the other way round. With a high ping, your hitreg still works just fine (as long as your ping is stable); you're much more difficult to be killed by the others though. I even made a video about it couple of years ago (unfortunately I cannot find it now), and it clearly showed that if you play with a high ping, it's perfectly playable for you, but painful for the others.
All that being said, I'm very interested in seeing some demos/videos of you cheating on the servers where you claim to have cheated.
Also, could you please tell us what cheat you based your cheat on? That would help us a lot, thanks