The distinction between "art" and "pornography" is also somewhat artificial. It's hard to argue that [3] was not meant to be prurient when the model (underage, by today's standards) shortly thereafter became a mistress of the King of France, based on him having seen the painting. And some stuff that was painted might get you banned from OnlyFans even today, e.g. [4].
But the problem, it seems to me, is that the internet has turned into a place where (1) everything has to be "safe for children" and (2) said safety standards are defined (through US influence, I strongly suspect) to be highly permissive of violence, but super strict on nudity.
[0] https://artsandculture.google.com/asset/the-war/CwHM2HdTO3l2...
[1] https://www.wikiart.org/en/max-ernst/the-angel-of-the-home-o...
[2] https://www.wikiart.org/en/matthias-grunewald/the-crucifixio...
[3] https://commons.wikimedia.org/wiki/File:François_Boucher_-_B...
[4] https://www.wikiart.org/en/jean-honore-fragonard/girl-with-a...
It is not just safe for children. Safe for work too. Safe for people who don't like porn in their feeds too. I think that these filters don't particularly care about high art, because that is insignificant percentage of overall nudity people post. Most of it is boobs and genitals.
The art you posted is all tame. But not all art is tame and like between porn, erotic and art is often blurry.
That said, someone will get upset at just about anything.
But in reality, people don't want to see erotica and such in their own feeds. The threshold of where it becomes unwanted is different for everyone. But most people want some level of filtering to be done for them.
Plus people want to be able to scroll Facebook or Twitter in work for few minutes without risking something inappropriate shows up.
And lines between art that feels good, erotics or porn, and basically bad art/photo that is super cringy just to look at and disgusting are blurry. And they are also subjective.