The picture-in-picture feature is only available on the iPad. PiP has been removed from the list of experimental features, where it was previously listed with an April 8th expiry date. The feature you are trying has been turned off and a banner notifying you that the test has ended, but both it and i More say that the feature is still working.
Picture-in-picture support is available for video services generally at a system level for both iPhones and iPads, but YouTube's support for the feature has been spotty, requiring workarounds like watching it through a browser rather than via its native app. The Premium test of the feature made it much easier for viewers to navigate away from the main YouTube app.
Premium users were always meant to be limited in the picture-in-picture test. After being announced in June last year, the beta period was originally set to end on October 31st. Although the feature was only for Premium users, last month it was confirmed that it will be rolled out to all users in the US in the coming months.
The end of the picture-in-picture test means that the video platform is ready to roll out the feature more widely. Last month, the search giant brought picture-in-picture support to its YouTube TV service. It is hoped that regular YouTube isn't too far behind.