Valve announced Dota 2 Reborn back in June, and released the beta shortly afterwards. Now it's properly, mandatorily here: the Dota 2 account tweeted the news yesterday.
We are rolling out the Reborn update over the next couple of hours. Matchmaking is now only available in Reborn.September 9, 2015
Reborn is a huge update that brings Dota 2 across to the Source 2 engine. Chris will have a more extensive breakdown in Three Lane Highway later on today, but basically it features a new UI, that new engine, and "new ways to play". Don't worry, that latter doesn't mean the game itself is changing, but there's a "Custom Games" feature that gives players the tools to create their own games—or "experiences"—for other people to play.
Tweet aside, it's been an unceremonious un-beta-ing of the beta—the switch mentioned only as part of the blog post announcing the Dota Levels update. For some players, it's also still behaving like a beta. Both framerate drops and unregistered clicks have been reported.