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).
It’s those few asshole cyclists that give all the others a bad name. Joe Cool whizzing past you and startling you ain’t that cool. Slow the fuck down with your branded “Hey! I got a cool bike!” bicycle. You don’t need to dismount though.