alt Hacker NewsOLED iPad dont have always on because of burn-in. Considering people certainly use it as photo frame, notification and time daahboars, kitchen recipe book, etc.
Less of a problem for iphones that unlikely to stay for a week in the same place plugged in and unused.
I don't think many people are spending $1k on an iPad Pro, the only iPad with OLED, to use as a picture frame.