Burn in and change over time. It's still a thing. It's still a problem no one has solved.
I haven't seen burn-in in a longggg time on OLED displays. Maybe if you leave the home screen on for months like a store model but even then the new panels are much better than the old ones. Not sure about "change over time", I haven't noticed it. OLED blacks are so much better than LCD, so much so that if you put them side-by-side that LED looks completely washed out. If Apple is looking into it now they must believe in it more than LCD too.