Oh wow I thought it was a limitation of the free version, but not Picture in Picture even with YouTube premium?
Welcome to Google not caring about iOS users, as always.
I bet if YouTube was still owned by YouTube, we would have had PiP eons ago with iPads, and had it with iPhones as soon as it was available. Now its all about the fact that they can't play ads or something, who knows. The most we get is the absolutely worthless feature, background audio, which all that does is waste my battery. Ugh.