Smelly plumbing mystery - what is this? (extra pics in comments)

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.”