These are the points of inspection set forth by the RMV under MGL C90 S7A:
"In addition to the emissions testing requirements, the rules and regulations for the periodic staggered inspection established hereunder shall include, but not be limited to, an annual maintenance inspection to determine the proper and safe condition of the following: brakes, stop lamps, lights, directional signals, horn, vehicle identification number, steering and suspension systems, glazing, windshield wipers and cleaner, number plates, tires, fenders, bumpers, external sheet metal, reflectors, splash guards, chock blocks, safety belts and exhaust system."
If a bumper is completely missing, they might gig you during inspection on missing body parts under the "bumpers or external sheet metal" parts, but as long as something is there and it’s not rusted out and falling apart, I'm sure you'll be fine.
Not sure how old you are, but years ago the rear bumper on a pickup was an "option" that you had to pay extra for when you bought a new vehicle. Many pickup trucks drove around with no rear bumpers at all back then. I've never seen any changes to the law regarding the bumpers specifically, however that was back in the day that ALL pickups had to be registered commercially in MA. That has since changed, and most pickup trucks may be registered as passenger vehicles if used for non-commercial use, unless they have dual rear wheels. Then owners are stuck registering them commercial. Hopefully if the State ever gets around to passing Bill S.1772 filed back in ’08, that will change