YouTube has been offering PIP mode for long, but it was only limited to users of its paid Red service. But last month, there were reports of picture-in-picture mode working for a limited number of non-Red YouTube users as well (both in the US and around the globe).
Now, it appears the rollout has expanded to everyone in the US. Needless to say, this is a good news as YouTube app is one of the most used Android apps out there, and the ability to have PIP mode working (both inside the app as well as when you switch out of it) is definitely a convenience.
Keep in mind that content owners on YouTube have the ability to disallow PIP playback for their videos, so don’t expect to have the feature running for all YouTube videos out there.