Valve dropped a giant banhammer on cheaters, resulting in a river of salty tears as well as a celebration by regular players.
Welcome to Valve Banhammer: 40,000 where Lord Gaben's armies of developers committed exterminatus against a cheating hive of scum and villainy.
The blog post on Dota 2 site noted that over 40,000 accounts were terminated due to cheating but the decision didn't happen overnight and the process of catching them was a lengthy one.
Namely, Valve identified a set of cheats players were using in the game and created a "zone" where they would expose themselves upon using illegal software.
The result is that Valve has an "extremely high confidence" that the bans were deserved because the honeypot nature of the trap meant only the guilty would fall for it.
In other words, when someone claims they have been wrongfully banned from Dota 2 in the near future, they are almost certainly lying to you.
Valve extended thanks to the people who helped resolve the issue, which included the players who reported others who acted suspiciously, which is probably what led to the discovery in the first place.
Just like any other competitive game, Dota 2 is at its best when the teams have a level playing field and after this ban wave, there should be a surge in match quality, unless matchmaking makes a mess of itself.