Since nobody on the internet has provided a screenshot of the feature, can you please at least say where the line appears (top of the screen), how many pixels wide (looks like ~2px on my 16 pro max), and for how long (forever, even on the always on lock screen)?
In the meantime, here's a couple screenshots for anyone who finds this:

(github won't let me upload directly for some reason)
https://imgur.com/a/RvQpTZQ