Update: Valve have expanded on their statement.
Lots of folks use Steam and lots of developers release their games on Steam. Thanks to the Steam Community in the Steam store, it's also a handy way for developers to let players know about upcoming updates or development progress on games that haven't released yet. Steam's new guidance to developers dictates that their Steam communication with players shouldn't mention availability anywhere else.
The change has shown up in the FAQ section for Community Hubs on Steam.
"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."
The Community Hub for Steam games is actually pretty broad, encompassing the News posts functionality that developers often use to distribute patch notes or development updates. It also includes the forum-style Discussions section where some developers run Q&A sessions with players.
The language in the new rule is pretty broad. This will almost certainly affect game News posts promoting secondary launches on new stores after a Steam launch. It could also impact smaller offenses like including links to all available storefronts for a game in a news post. The new rule also specifically calls out content patches that might exist off of Steam's store. It's not clear just yet where Valve's enforcement focus will be.
Update: Valve's Doug Lombardi has sent us a statement saying, "the general spirit of this update was to remind content creators that their Steam pages should not be used for certain activities such as for the promotion of a game's exclusive availability on a competing platform, the promotion of an external download that circumvents Steam content policies, or the promotion of other activity that conflicts with the Steam Distribution Agreement. The new language on the FAQ was not really the introduction of any new policy or policing that should concern the majority of those publishing on Steam, but more of a reminder of existing rules for a small number of developers exploring the boundaries of the existing policies."