There are some hypotheses about this but not a lot of evidence to support it. Yeah, if you didn’t interact with other ppl for some time, you may get a cold or other virus when you start interacting socially again, but that doesn’t mean that the mild infections protect against more severe ones down the road. Immunity is largely pathogen-specific, and even for the non-specific part of the immunity, it will develop regardless of having contact with actively sick people at a bar. Preventing obvious exposures shouldn’t be discouraged— you’ll get exposed to stuff anyway.