No anti cheat for FPS games has ever "worked", it can't. The best you can do is make it a little hard for the cheats to keep up with your detectors or protocol changes.
That's absolutely true! Valve anti-cheat has entirely failed at that. Free open source cheats exist that VAC just cannot detect. Period. Wanna know the secret? The fact that it's a Java cheat.
Some speculation it is Java, some that it is Java's license, some that it is the license the cheat is under (open source), etc. No one really knows why.
Strangely the only difference between one java cheat that was detected and one that has been undetected for four years, is that the original, old java one that got detected was licensed upder GPL, and the newer one is licensed under AGPL. Then there's a newer fork with a GUI that is undetected for ~2 years.
VAC seems to be... unable or unwilling to detect Java cheats. The original, old one got detected, though, and it was Java. so there is a tad of confusion.
I have sent countless messages to valve offering patches for several current exploits, like the current server lagger/crasher that allows teleportation. They literally just do NOT care. At all.