There are many cases of casual fixe matches and unfortunately it is difficult to prove it.
Valve stepped down and is giving a very tough sanction for those players and teams that will be proven to be involved with a fix match. They have already made a sample for those players that did it in the past and they're feeling sorry about it because Valve events are the biggest prize pool tournaments that they can no longer participant.
Sample of teams and players that were banned by Valve.
https://liquipedia.net/dota2/Banned_playersOkay, now tell me, how do they get to this? after all, this cheater will always be one step smarter, and I can't imagine it's even 50% detectable. How many matches like this are actually caught? Detected ? 10 - 20% max ? Generally we do not know.
Severe punishment ? I don't think you know what kind of money can be behind this harsh punishment... It may not even be half a game...