Uh oh for naysayers....and I was one of those doubters. I did a search on my manual and found this page. Suffice it to say, I will HAVE to test this out. For reference, I'm in New York, USA.

https://cimg6.ibsrv.net/gimg/www.mbw...086db6abbc.jpg

Same page #78 '19 manual konigstiger posted in post #7 above. As myself and an other poster stated above, it does work as specified in the US manual. "It" refers only to when roof is raised in vent position (not opened) and only while driving. Unlike EU/other markets W213s, US' do not close opened (not raised) sunroof when it starts raining while driving or when car is parked and locked.

The key words are "while vehicle is in motion". Others are saying this functions when their car is stationary.

I am confused about one thing: does it only apply to the panoramic sliding sunroof, or all sliding sunroofs?

Yes, pano only. Regular sunroofs, non-pano, work as always (if it rains, driver has ability to still drive with roof in vent position - rear raised).