Just look on FlightAware or similar. They've all got the historical record for a given route (though of course it's possible an airline might substitute after you've bought the ticket, it's unlikely; most flights fit a specific profile and most airlines have only a couple of models to fit that profile).

The major users from North American airlines are Southwest (34 planes), Air Canada (24), American (24), United (14), and Aeromexico (6), so it's not super common here at least.

I strongly doubt the overwhelming majority of consumers are going to cross reference booking details against an external site to avoid a specific plane unless there are planes falling out of the air on a daily basis. People may say they will, but in practice they'll look up the cheapest flight and book that, as usual.

Depends on how prominently the information is displayed, and the price difference.

If I can see the plane type in the list of available flights, and the difference is $15 on a $250 ticket I would choose the non Max flight, all else being equal.

Fair, and someone mentioned that Kayak does display that, but I think you will probably have to move to a different airline to get a different aircraft type (airlines use one common type for most flights on a given route) and the price differences are often steep and/or there are other sacrifices to be made (frequent flyer miles, more expensive bag checkins, etc).

I don't disagree at all. The information's there, though.

Aircraft type is even a filter option on some travel sites, like Kayak.

It’s listed on several UIs for purchasing tickets, but usually it takes several clicks to identify the model of airplane.

During the reservation process you should be able to see the equipment type (that's airline-speak for "model of plane").

It might turn out to be a competitive advantage for the booking site, who might then highlight "not a max" in the process.