Still waiting for my iPhone X to come in but from reading their info, they say burn in is a result of brightness + duration. From that it seems that the black strip will not be an issue.
Black elements absolutely burned in on the S7 edge my work has for app testing. And this is a device that, obviously, isn't even anyone's daily phone, nor does it have any reason to be "left on" for super long periods of time when it's not in someones hand. Those areas end up brighter than the rest of the screen, rather than darker. It's sort of a reverse ghosting.
I ran one of those "burn in fixer" solid color pattern apps for almost 24 hours(which i know have hit or miss efficacy) and it subdued it, but it's still very clearly visible.
I ran one of those "burn in fixer" solid color pattern apps for almost 24 hours(which i know have hit or miss efficacy) and it subdued it, but it's still very clearly visible.