In my experience, KGS is nowhere near as popular as it used to be, sadly. Tournaments dried up, player base dwindled and it's not the same

How do you cheat at Go? The only thing I can think of is cheating by self-playing thus gaining illegitimate rank?

Just like with chess, when AI engines got very strong a few years ago, people started using them to find superhuman moves and win games. Now OGS will tell you how similar your game's moves are to what a computer would have played. Even professionals don't get above something like 75-80%, but I've lost a dozen games to people getting 95%+ on that metric.

You could cheat in many ways.

One way might be to use a Go engine to analyse the best moves for you as you play. Even for non online play this is already a problem with Chess games and hidden smartphones. Everyone has a 9d bot at their disposal these days. And of course one could make use of cloud TensorFlow if that’s not enough.

There are ways of detecting cheating.

Main way is using an AI to tell you what the next best move is.