We are rolling out the Reborn update over the next couple of hours. Matchmaking is now only available in Reborn.
— DOTA 2 (@DOTA2) September 9, 2015
Dota 2 has officially been reborn. Starting now, you'll only be able to play Valve's popular MOBA in its revamped form, complete with the sexy new game client. The game is also adding a new achievement system called "Dota Levels,".
You can read about the new Dota Levels system here.