Not just CRTs - I had an LCD around 2006 that I left on a browser window for hours and I had the browser's window & menu bar slightly burned in for a good few months after that.
Yep, there are two different 5K panels on iMacs, off the top of my head LG and Samsung, and the LG one does have slight transient burn in.
It's not "true" burn-in (like CRTs or OLED) as it goes away after a bit but it's fairly easy to trigger with the current trend of very white/light grey designs in macOS, the easiest way to notice it is move from a whitish area to a mid grey one (Daring Fireball is a great example of that problematic grey).
I also have been unlucky on that draw on my 2015 iMac. I barely noticed it at first but newer macOS versions have made it very noticeable sadly. One more reason to run Aerial ;)
