Live streaming support for third-party apps is among the 30+ developer-focused features Apple did not discuss publicly during the WWDC 2016 keynote. Realized through ReplayKit Live, a new framework on iOS 10 that lets players share gameplay recordings or broadcast live games to other players and viewers online, the feature was originally absent from the first beta of iOS 10.
iOS 10 beta 2 turns on this functionality and, as a Reddit post noted today, live-streaming can already be found in Apple's Swift Playgrounds app for the iPad.