I would love this feature too.
If this were implemented, it would be great if it weren't just implemented for "Full Screen" or "Focus" mode. Some applications will allow you to hide distracting elements, but only in a "Full Screen" or "Focus" mode. The problem with that is that many of us use windows side-by-side, not full screen, for instance if you have a large monitor, since most large monitors these days are widescreen.
@Vadermaus, another option would be to use an app that lets you pin a window on top, and just stick that window over the distracting element. I use and love PNotes for this, although there are lots of apps that have this functionality.