I remember encountering CRTs burnt in so badly it was hard to read stuff in the worst areas (e.g. the taskbar clock or login prompt) but I haven’t encountered anything remotely close to that with current OLEDs. My iPhone and TV have no signs at all, and the last device I used that had legitimately easy-to-detect burn in was a Nexus One test device that sat on my desk with the screen on all day every day while I built an Android app in 2012.