I've heard all sorts of stories, from paid Envoy being bumped to Y, top-tier elites on certificates or miles upgrades bumped to Y, and so forth. I don't think there is any "right" answer as to who to bump--- is the customer who pays for Envoy but only flies once or twice a year "worth" more than the passenger who upgrades using miles earned after flying 100k miles a year? To be honest, there are so many stories of people being angry at being bumped regardless of how they got that Envoy seat, that I don't think US has a SOP, or if they do, they don't always honor it.

To that end, if I were worried about being bumped in an equipment downgauge, I'd select seats that exist on as many variants of the plane as possible, or at least the "usual suspects" one could expect to see on that route, and check in online as early as possible. I think identifying potential bumpees is done at least in part by computer, as it's easy for a computer to identify a passenger who does not have a seat assignment.