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.
Religion is not “invented”, does not exist
People have no imagination, or have literally all the answers and thus no free will in this world then?
Can’t imagine a world where people don’t create incorrect answers associated with a higher power