Apple have stuck to LCD for all their iPhone's. The iPhone X will be the first with an OLED panel so it will be interesting to see if after a few months it also suffers from burn in.
Apple tends to be willing to pay more for the parts and they have very high quality standards.
That said they’re not immune. There have been a number of displays Apple used in the last few years with retention issues (I think the original rMBP has issues) so it’s perfectly possible.
If they made the gamble they must think it’s ok but as someone who plans to buy an X I can say it does worry me a bit.
Edit: another comment points out their keyboards. That obviously hasn’t been going well either.
The problem with the Pixel 2 XL screens seems to be attributable to LG. LG's own V30 phone has similar screen issues. Meanwhile Samsung's OLED panels are doing great so far - and Apple is sourcing from Samsung.
I've never seen a 1st gen apple watch with burn in, even. Mine is a bit over two years old. Apple seems to buy the absolute best quality displays, and have good control logic/orbiting/etc to prevent it. I use mine heavily, although i guess not as much as a phone. I've never heard of one getting burn in/retention though.
