Steam has taken a strong stance against forced advertising in games. While the mobile gaming industry may rely heavily on free titles that force you to watch ads between levels, Steam doesn't want to go that route, and has effectively banned games from using advertising in this way.
"Developers should not utilize paid advertising as a business model in their game, such as requiring players to watch or otherwise engage with advertising in order to play, or gating gameplay behind advertising," reads a new webpage from Steam. "If your game's business model relies on advertising on other platforms, you will need to remove those elements before shipping on Steam."
Things like microtransactions, product placement, and cross promotions are still allowed, but developers are forbidden from trying to providing value to players via advertising. Nor are they allowed to charge other developers for access to Steam's features.
As per SteamDB, these rules aren't exactly new, but now they are outlined explicitly with their own page, so that developers know what they are and aren't allowed to do when it comes to advertising on Steam.