Valve has updated Steam's "Rules and Guidelines" for Steamworks Documentation, which are the rules of the road if a developer wants to upload their game to Steam. The new update adds a new 15th clause that prevents a specific type of game from being listed on the biggest PC gaming storefront.

Before the update Valve listed fourteen clauses that developers have to adhere by to get their game listed on Steam, and now there's a new 15th clause, which states that developers are not allowed to have their game contain any "Content that may violate the rules and standards set forth by Steam's payment processors and related card networks and banks, or internet network providers. In particular, certain kinds of adult-only content."
Following this change, many games and DLCs have been removed from Steam as they are now in violation of Steam's new guidelines. Screenshots have been provided below of a selection of titles removed, and notably, the majority of the titles that have now been deleted from Steam are in violation of the adult content policy.

Judging by Valve's wording in the Steamworks Documentation, it appears payment processing providers, such as Visa, Mastercard, or any other method of payment currently available on Steam, aren't happy with the adult content on Valve's platform. There wasn't an explanation as to why they aren't happy with it, so we can only guess that the companies, at a minimum, don't want their service associated with this type of content.
According to SteamDB, a website that tracks Steam movement including updates to games, players, rising titles, etc, "a large number of games has been removed from the platform in the past 16 hours. Judging from the list, it appears "sex simulator" type games with keywords such as "incest" and "slavery" make up the majority. There are some confusing cases like the removal of the Ace Attorney-inspired investigation game Trails of Innocence, although this could be a coincidental deletion."
In a nutshell, Valve had two options to choose from. The first option is to keep the content on the platform, risking the payment processors removing their service from Steam. The second option is to remove the games containing the content that the payment processors are unhappy with. It appears Valve selected the second option.




