Actually i notice that as well ether some of the people have fond a loophole on of adding games like that on steam or they change there policy, the reason i mention that steam doesn't accept naughty games like this (i hope nobody shoot me for this) i purchase some visual novel on steam the novel sold adult patches as dlcs well the recent novel i purchase from that series did not sell the adult patch when it was release so i look at the forum it turn out that steam doesn't accepted adult content anymore they change there policy, anyway it look like they change there policy again or they just don't know anything.
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Well for the current state of steam policy that part i have know idea where to find that but there might be some info on the steam forums.
And as for the some 18+ games you are right that some devs give patches that you can download in the devs website or some 18+ forums or paid for the patch on some other website other than steam but lately some devs put 18+ without censorship on steam this is very weird after i mention about the policy.
Normaly the policy is that if the game has characters looking like kids in them, they not allowed on steam unless they removed from the game.
Alot of games with porn excist on steam, and can be allowed. Only games with characters looking minor or child like, like in those VN's like Nekopara are normaly not allowed on Steam, because the original game of it is NSFW and the characters look minor, so they have to remove that content out of it to be allowed to stay on steam.
Found a few of what they dont allow regarding 18+ content
Sexually explicit images of real people.
Adult content that isn’t appropriately labeled and age-gated.
Content that exploits children in any way.
Since all these has nothing to do about this game, it is allowed on Steam as long they label it and age-gate it correctly. But it can be the publishers cant allow it to be sold on steam unfinnished.