I also see that line on my 13 mini (but not on my updated iPod Touch).
I also assume it’s a hint to swipe-down from there for the Control Center. I noticed that swiping-down from other locations does other things, so it may be a useful reminder for some people.
I never noticed it before on my 13 mini. But I just did a search and I see it in a few screen shots you shared in a post from 2023:
Do You Use It? iPhone Always-On Display Popular but Not Universal - #9 by Simon