Once upon a time, no visual novels were allowed on Steam. Then the first few tricked in and people started to push the envelope.
Valve initially took the stance of ignoring companies advertising their adult patches in the Steam Community forums and eventually seemed to accept it. MangaGamer, Frontwing and others finally pinned articles about how to download 18+ patches for various visual novels.
Then, sometime between September and October, Valve issued a policy which put an immediate stop to this behavior. Visual novels which happen to offer adult patches are still allowed on the storefront. Developers/publishers can no longer mention these patches anywhere on Steam, however.
This means visual novel fans will need to find where to grab patches via Twitter, Patreon, Kickstarter or other locations outside of the Steam storefront from here on out.