I see these every time a bicycle path intersects with any kind of car or train infrastructure. It just seems really patronizing, as if we’re not capable of slowing down without getting off the bike; it also shifts responsibility onto the cyclists if there is a crash based on something arbitrary (mounted vs dismounted).
I’ve never seen such signs, despite riding my bike a lot. Isn’t it just a suggestion though? I never dismount.
Maybe it’s to reduce the suicidal behavior that a small percentage of cyclists have? When I’m driving for example, I always give cyclists right of way on crosswalks - despite that the law here is that cars have the right of way (unless dismounted). And yet, I see people all the time who don’t even look, just riding at really high speeds right over. I really don’t get it.