This is an experimental YouTube feature, so it's not something you'll find on your phone unexpectedly in an A/B test-like fashion. Instead, it's a YouTube Premium perk. Premium users can opt in to try it out and give feedback to Google. The feature is also only available for a limited time — you'll be able to use it until September 1st, at which point it will likely be removed. Afterward, Google might decide to roll it out as a feature for everyone, or it might opt to scrap it completely.

If you want to check it out, make sure you're subscribed to YouTube Premium (which will also give you ad-free videos, downloads, and background playing on both YouTube and YouTube Music). If you don't feel like subscribing, you'll need to wait and see if Google rolls this out to everyone once testing is finished.

Source : OnLeaks(AndroidPolice)