A clause in the newly-released Steam User Agreement now requires people to disclose when they have been given compensation for promotions or endorsements.
Located in section 6, paragraph C of the , the new addition reads as follows:
"If you use Steam services (e.g. the Steam Curators’ Lists or the Steam Broadcasting service) to promote or endorse a product, service or event in return for any kind of consideration from a third party (including non-monetary rewards such as free games), you must clearly indicate the source of such consideration to your audience."
Valve added a similar rule to their Terms of Service back in 2014, making disclose if they had been paid by a developer to put games in their lists.
In related news, Valve recently addressed the by the Better Business Bureau.