Valve banned more than 40,000 Dota 2 accounts this week, doling them out to players who were found abusing the matchmaking system. The specifics of the ban wave weren't made clear in Valve's tweet, below, but smurfing seems like the most likely suspect.
We have banned over 40,000 accounts for players who were found abusing matchmaking. These bans will now appear as game bans in Steam as well as being matchmaking bans in Dota 2.February 11, 2020
Experienced players making additional accounts so they can beat up lower ranked players has been a perennial problem in Dota 2, and plenty of other games with matchmaking, and the recent bans coincide with a game update that tackles the issue.
"We are making our smurf detection system more sensitive in this update," Valve says. "This change will much more proactively target potential smurf accounts, but may on rare occasion give a normal player extra MMR."
Valve often targets large numbers of smurf accounts, like the 17,000 it banned in January last year. The accounts were found to be abusing matchmaking to get into ranked. In 2019 it also introduced changes to how the matchmaking rating was calculated and the aforementioned smurf detection system.