Earlier quoted context omitted.
I visited a TV shop about 5 years ago and some of the TV's up on display had the demo image burned in. So it's probably still a problem.
From what I have heard this is a problem with OLED panels.
There is also a navbar (back/home/recent buttons). In my case it's a white-ish rectangle that's almost always present on screen. By the time Samsung added an option to hide it, the damage was already done.
I have them both burnt in on my screen. I first noticed them after merely 1.5 years of using the phone.
OLED burn in is a 100% real thing.