It looks like YouTube is done extending its picture-in-picture test for Premium iOS users. It has already disappeared from YouTube's list of available experimental features. It was previously listed with an April 8 expiry date. Some users already see a "The feature you're trying has been turned off," but it looks like it's working for some accounts who previously signed up for the test.
The test was supposed to be for a limited run anyway. But YouTube has said that it plans to bring picture-in-picture to all iOS users in the US, even the non-Premium ones, "in the coming months." Now, we're hoping that update also makes its way here. We'll keep you posted on any news!