I think jokes can act as anti-shibboleth. There are some jokes that are so obvious and overused that it stops being funny to insiders and they moved on. I am thinking of something like "Uranus" -> "your anus". I may be wrong with my example but I suppose astronomers like toilet humor as much as anyone else, but at some point, they need to do their thing without getting constantly distracted by that planet name, so they just stop joking about it.

It is not always obvious which jokes are off limits. Because they may not be bad or offensive, it is just that they interfere with day to day activity, and outsider may not know what are these day to day activities and how disruptive it is.