While it’s not the only criteria, it is almost certainly a factor. Delta I think had the least amount of involuntary boarding situations last year (I think I read that) but I would think, given that the fare paid helps to determine the amount of compensation owed in a situation of involuntarily denied boarding, those paying the lowest fares would be among those at risk. No airline that I know of publishes their actual procedures for this but I’m just guessing those with status, paying full fare, business and first class are at less risk than a basic economy ticket. Just my opinion, really but that seems logical to me.