It's more of a philosophical question. I know that the "grading list" of passengers to be operationally upgraded in oversell situations is entirely system-generated, based on several parameters and weightings. But somebody's got to input these parameters in the first place.
One of the parameters, FWIW, is a special meal request. Since no special meal to C standards would have been catered, Y pax with a special meal are deprioritized on the grading list.
As for the frequency of op-ups, there is a route and time that I used to fly regularly, with an exceptionally good op-up rate. High leisure, low business routes / times are the key.