I have been thinking about how to implement that after the MAX starts flying again. Flight search interfaces don’t seem to filter on equipment. And of course, equipment gets changed at times, so you may book one aircraft and have a different one swapped in on the ramp. I wonder if after you have checked in, and the airline swaps in a MAX, if you can get credit for your ticket if you refuse to board a MAX? I would guess not. (Yes, I know airlines try to avoid changing equipment because it causes reseating hassle and crew scheduling hassle. But it does happen)