no explicit art pieces in mainstream "real" museums, censored or not. R-rated is the limit.

explicit can be seen at adult museums with age check. it's not censored. but the "art" could be imitations by modern artists or cheap/unauthorized (photo-)copies.

adult museums don't receive respect in Japan so they can't get get whatever approval to buy real historical materials.

TL;DR - no matter how much Japanese filth you find on the internet, daily life in Japan society is extremely prudish and anti-sex. part of the reason there is so much Japan porn is because nearby countries (China, Korea, Vietnam) ban porn totally. so so many customers download Japan stuff $$$$$$