Is there any way to set up a divinely mandated masquerade in a setting involving deities from historical pantheons? The myths I’m going on don’t have a precedent for non-interventionism, as (for example) the Greek and Norse deities regularly got involved in mortals’ lives. I’m also not sure if it’d make sense for deities to follow a course of action leading fewer people to believe in them. I’m thinking of either going with something about free will (like you suggested in your post on explaining the masquerade), or coming up with another cosmic entity to set that rule. Thoughts?
Yes, I definitely think you could use Greek and Norse deities to set up a masquerade. I wouldn’t try to use the same explanation that I would use if there was a deity inspired by the Judeo-Christian god, like free will or non-interventionism. The gods in old pantheons tend to be depicted as very petty and willful, so if I were to use them, I would look for an explanation that fits the personalities they have in mythology. For instance, maybe the gods (or just their leader like Zeus) are still mad that Prometheus stole fire from them and gave it to humanity, so they have decided to punish humanity for the crime by denying them magic.
If you simply establish that the gods aren’t any less powerful if humans don’t worship them, I wouldn’t worry too much about them doing something that makes humans believe in them less. For them, making it so humans don’t believe in them could be like giving humans the cold shoulder.
Then you just have to make sure your magic workers in the story are an exception to the rule. That could easily be done if they are descendants of the gods or another nonhuman from mythology, like the Titans.
The other great thing about using a pantheon from mythology is that the gods are not uniform, and they often bicker. So while everyone generally would try to follow the rule to avoid a terrible fate, you could easily have some gods try to break the rules without getting caught, if that fits your story.