Valve Doubles Down On Banning Forced In-Game Ads In Steam Games
Strictly speaking, Valve has always banned the most popular mobile-style monetization schemes. Those are the "in-game ads" type discussed above, and the "paywall" type where a customer gets blocked after a certain amount of playtime and has to pay to continue and keep their progress. These types of monetization are considered predatory and are not allowed in Steam games.
However, Valve recently created a whole separate page in the Steamworks developer documentation to clarify the issues. The digital delivery service explicitly states that developers "should not utilize paid advertising as a business model in their game," and that for developers of games that work that way on other platforms, "you will need to remove those elements before shipping on Steam."
Furthermore, Valve has also explained that developers can't ask players to watch advertisements for in-game rewards. This sort of thing still goes on in other places, like Twitch streams that reward players with in-game items for a certain amount of viewership (known as "drops"), but Valve is putting its foot down where it has control, at least.