As has been said before, many UA 737 Max aircraft do have airbag seatbelts, at least in the bulkhead row of first class. They do allow use of an extender (it basically deactivates the airbag when an extender is in place). However, note that the extender necessary for the airbag seatbelts is different than the "normal" seatbelt extender. They stock both on these aircraft. If you get the "normal" type and are in a seat with an airbag belt, it will fit but not latch on the airbag side. Ask the FA to find the correct one for the airbag belt. It may be in the demo kit rather than where it should be in the storage compartment in the bulkhead wall. The airbag-compatible extension has a more squared off, larger opening on the insert side. For Polaris seats, the "normal" extender works and there is no special one needed.