Valve have added a new rule to the Onboarding guide for game developers, noting that payment processors get a say in what stays on Steam.

Newly added rule is:

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.

Diff of the new terms https://github.com/SteamDatabase/SteamworksDocumentation/commit/fddd59b5395cc3c1c74574650dbf5784612d0521

:/ payment processors strike again (slippery slope etc)