The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
Maybe instead of schizophrenics talking to the dead they would just fit into the class of spiritualists and diviners, oracles and the like.
I’m open to interpretation.
After a little more thinking on this:
Gender dysphoria? Nope! Shapeshifter. Furry? Nope Werebeing
Basically, anything that is a deviation from bog standard human would have a magical reasoning behind it.