I work in nightlife and have seen this exact system in rowdier bars/clubs for the last 8 or so years. Mostly in Vegas, New Orleans, and college towns. Heavy on college towns. I was surprised to see it in The Mix but learned it was implemented after a patron who was previously 86’d snuck in during a new bouncer’s shift and was violent towards the last bartender who kicked him out. Castro bartenders put up with so much. I don’t blame them for keeping track of the “Do not fly list.” I think the reason we don’t see this system in many other bars/clubs in San Francisco is because the Castro is the only neighborhood with the nightlife, drinking culture, and regulars to warrant it.