Don’t really think guests would be down with a bathroom window you can’t close curtains or a blind on.
A shelf could be attached to the bottom piece of the window frame, though. That woulld be high enough above the toilet flusher and still allow for the window to be opened and the blind accessed.
But I’d say if RiverRock, who I think has been hosting for a long time, has never had a guest complain about not having space to put their toiletries, it’s not worth fussing over putting up a shelf if there’s really no viable and logical place to put one.