https://techcrunch.com/2018/06/29/youtubes-picture-in-picture-mode-is-live-for-all-u-s-android-users/

YouTube has confirmed that picture-in-picture mode – previously a paid-only feature – has now rolled out to all U.S. YouTube users on Android on supported devices. The feature, which works on Android 8.0 (Oreo) or higher, had been slowly rolling out to non-Premium subscribers since this April we understand, and the full rollout had completed on Monday.

The website XDA Developers was the first to spot the expansion, noting earlier this week that it seemed as if picture-in-picture mode had suddenly been turned on for several more YouTube users. They suspected that this had been a gradual launch, but didn’t confirm the status with YouTube directly.