In any case you speculated that Steam might be trying to clear porn games from the platform in your initial comment (or inferred such) and one game doesn’t validate that claim.

Quite the opposite. The reason I suspect there’s something legal behind behavior like this is that it is so laser targeted to this game. Especially when it was immediately followed up by their competitor eager to host the game (which had already removed the content named in Steam’s initial reason) and then changing their mind at the last second.

What I see in common between Horses and Github is that it appears that they see it as a bad idea to explain publicly why they’re doing what they’re doing, and that smells like a legal reason to me.