My 2020 CX-5 has that extension at least on the drivers side. I don't recall if there is one on the passenger side, I think not, but I'm on a road trip in the other vehicle so I can't check.I had a Saturn with visors that had a sliding extension that helped block the sun behind the mirror. It could also be helpful when used to the side if necessary.
On of the youtube reviewers mocked it, I think because it is a cheapo solution. I've found it useful at times, better than nothing.