This is key. I do a lot of world building beyond what the PCs will every encounter because A) I love world-building, and B) because a deeper setting usually facilitates a better campaign and more immersion (i.e. it avoids the feeling of "cardboard town buildings" ala old Westerns).....but unless those rules and truths are needed by the characters, they are not set in stone.
But...even then, if the PCs haven't interacted with it, nothing is set in stone. I will make adjustments that serve the Prime Directive of any campaign: to maximize enjoyment for the group as a whole (including the DM).