The only multiplayer game I currently play is Beyond All Reason (a RTS game).

It's a free and open-source game, so creating a cheat client would be especially easy. But I've never encountered cheating.

I think there's a few reasons for this:

1) The playerbase is small and there is no auto-matchmaking, just a traditional list of servers. This results in the same group of people always playing together. People don't want to cheat when they're playing with acquaintances they see frequently.

2) Spectators are allowed in every game. The top-ranked games usually have several spectators.

You might think this would result in even more cheating, but in practice the spectators would prefer to watch a sneak attacks succeed, because it's funny. It's boring to be whispering the enemy secrets to you buddy on a private Discord, it's more fun to watch your buddy die in a surprising and funny way.

Also, the spectators can spot if a player does something that suspiciously well timed or lucky. The spectators see all, so they have the information needed to spot suspicious behavior.

3) Official servers create an official record of what happened in every game. The entire community has access to all the recordings. If someone thinks cheating is happening they can link to the official game recording on Reddit (or whatever) and everyone can see what happened.

4) An active moderator team reviews every report of cheating. There are official moderators that do the banning, but also volunteer moderators which can watch the recordings and create a trusted written account of what happened; this makes the official moderators have an easier job.