What does that small line means on my iPhone X?
I've noticed it some months ago, so it isn't the first time I see it. 
It looks like its just for aesthetics
It's just a reminder that if you swipe down, you'll open up the control center. Before the X you swipe up to open control center. With the X and beyond, you swipe down to open the control panel.
It's supposed to be animated but you probably didn't notice it when you first set up your X. If you do a total reset, you'll see the animation telling you to swipe down.
Sorry, i have no idea
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