Millennia? No. They're about 70-75 years old tops.
There's a provision in the Japanese constitution/revised law codes, which was co-authored by 1950s America (y'know, when we had the CCA and Hayes Code and just saying "pregnant" on television would get you canceled) that forbids "obscene materials," which went on to be codified in the actual law codes as an anti-porn measure. So technically even censored porn
But here's the thing about Japan: They're more for social harmony than the actual law. They'll let a lot of shit slide if it's making people happy and not hurting anyone. It's the entire reason comiket is allowed to exist even though 90% of the products there violate copyright laws to say nothing of the obscenity laws.
So mosaic censorship is the "Gentlemen's Agreement" that prevents crackdowns. The last time someone pushed the envelope too far, they removed porn from convenience stores. Which...honestly I kinda agree with because that shit was sitting out next to the shonen jumps.
Oh also that obscene materials law? It's so incredibly vague it's actually hilarious. But there's not actually a provision against owning it. Just producing, purchasing, and distributing it. Mostly as a means to protect people who inherit it or get sent it against their will.
Japan actually used to be pretty egalitarian about nudity, and still are in some ways. Public bathing is normal. There is such a thing as non-sexual nudity to them. And until foreigners started popping their monocles in disgust and refusing to do business, farmers would do their farming NAKED in summer.
