I remember when iphone 4s came out in 2011, my iphone had some minor burn-ins. The top status strip had some mild burn-in due to constantly displaying the time, battery status, carrier info, wifi etc. That was 7 years ago. Over the years, Apple improved on the burn-in issues and now I don't even remember when the last time was when they completely fixed it.
OLED displays are more susceptible to burn-in than LCDs but I expect this year model to be more resistant than last year and next year to this year and so on and eventually in 3 years, or perhaps 1-2, knowing how Apple improves on their products' shortcomings every year and the fact that iphone is their biggest cash cow, burn-in will no longer be an issue if you give it time. All technologies mature with time.
My educated guess as to why Apple started making OLED iphones much much later than Samsung did was due to burn-in and that's why I feel they've been so conservative in doing so. I can say for a fact that OLED displays are eventually coming to macs b/c it is the next undisputed display technology.