Our laundry room sink has this bizarre attachment to to the back of the drain line clean out. It would almost seem like it was or is supposed to be a vent. Recently it’s started to give off a foul sewer smell when our washing machine rinses.

Can I safely plug it? Maybe remove it if I can manage to pry it off? When I asked our plumber what it was, he just smirked and said it was a “crazy thing.”