For reasons completely unknown, Valve has put in a policy change that seemingly prohibits developers from mentioning other platforms that their title might be available on.
In the Steamworks documentation, under the "Steam Community" section, there is a FAQ that has a question about developers using the Steam community to let customers know of non-Steam versions of their games. The answer given is below:
In the game you ship via Steam, and in communications on Steam, you may only promote the Steam version and its availability via Steam, and not other distribution outlets. This applies both to full versions of your game and to content patches that change the existing version.
This particular policy would seem to prohibit a developer who was working on a game that they were planning to make it available through Epic Games Store, GOG.com, itch.io, Google Play, the Apple App Store, EA Origin, or the storefronts for PlayStation 4 or Xbox One. It would also seemingly prohibit mentioning that specific titles were ported over from other platforms, such as Halo: The Master Chief Collection (from Xbox One) or Horizon: Zero Dawn (from PS4).
Steam users make use of Community Hubs to handle discussions about a given title, as well as putting up fan art and screenshots or links to gameplay videos. Developers and publishers make use of them as an adjunct news channel to reach fans directly without necessarily putting up dedicated news pages or blog posts on the game's website.
It seems a highly unusual policy to be adopted, and one which doesn't seem particularly enforceable, or at least not with any degree of uniformity, which wouldn't harm Valve's interests in some fashion. We've reached out to Doug Lombardi at Valve for further clarification regarding this policy and, as of yet, have not received word back from him. We'll update this story as more information becomes available.