If true, lockdowns would have had an effect on those without an appendix?

It would be interest to see how historically contact with X other people has changed over time. I don’t think you need a big city to mix with a lot of people. I imagine in older times there are fewer people but likely to be closer to each other - people sharing beds (not stupidly associating bed with sex) and tribes of families mingling close lower instead of nuclear family. By contrast the city apartment is the hermetic seal around a small number of people choosing to live together. But a modern concept: corporations, jobs and careers ensure people keep mixing up with people outside.