It seems the Fan Disc is on there, but will need a patch. The 18+ DLC that is on there for the base game would probably be banned if it came out now. Valve is a lot more strict when it comes to grey area underage content, school setting content, and other things.well the main gave had the OK for get the 18+ patch on steam, would be very strange that the fan-disc 18+ cant get to steam.
It's like the Nekopara 4 18+ DLC got banned on Steam, but the 18+ patches are for sale for Volumes 1, 2 & 3.