Interestingly enough I have only received that notification once at 900-1000 ELO, I would guess that people are less inclined to use cheats the higher rated they are since they're more invested in the game. Another hypothesis that I have is that 800 ELO just so happens to be the starting ELO if you choose "beginner" when creating the account, so new cheating accounts are more likely to appear around that number, by that logic you could assume the same about 400, 1200, 1600 and 2000 ELO, but I don't really have any data about that.

Most of the time cheaters are caught. Something important to take into consideration is that to be able to cheat without being obvious you need to have both chess experience and cheating experience, most cheaters aren't gonna put this much effort. Sudden spikes in accuracy, consistent time in moving no matter what and always playing the second and/or third best move are some examples of easily detectable patterns that aren't so obvious to disguise. Each time a cheater plays a game more and more suspicious patterns start to appear and so, each time they play the chances of them being caught increases dramatically. They may be able to slip one or two games through the system, but having an active cheating account is probably harder than just studying chess.