I have it on my iPhone SE, which I bought about six months ago. The line is about 1/16" inch thick and looks a bit like something behind the screen. It goes exactly from the lower left corner to the upper right. It may have been there all along, as it's only visible on a bright white screen. I noticed it today when upgrading my OS. The phone was sitting next to my computer with a bright white screen after it restarted.
After seeing photos of other peoples' phones, I'm concerned it may get worse with age. Luckily I have AppleCare+, so a replacement will be free.
I must say I've gotten my money's worth with AppleCare+. First my 4s went into the wash the same week I bought it. Then my 5s screen went crazy just before the extended warranty expired. The power in my Apple Watch went squirrelly a mere two months before the extended warranty expired. Mind you my stuff gets the best care possible. Always a case on it and also a screen protector on the iPhones and iPads. They're never where they can get wet, not even into a steamy bathroom. You'd be hard pressed to tell all my iStuff isn't brand new.