Did they? Just searched the web and, voila you are right: https://www.tomshardware.com/video-games/pc-gaming/valve-clarifies-ban-on-in-game-advertising-on-steam Now they do not link to the page they cite (bad habit, I hate that), so let me quickly find the source... aaand there it is: https://partner.steamgames.com/doc/marketing/advertising

Not Supported

• 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. If your game's business model relies on advertising on other platforms, you will need to remove those elements before shipping on Steam. Some options you could consider include switching to a single purchase "paid app," or making your game free to play with optional upgrades sold via Microtransactions or Downloadable Content (DLC).
• Developers should not use advertising as a way to provide value to players, such as giving players a reward for watching or engaging with advertising in their game.
• Developers should not charge other developers for access to Steam features. These include sale pages, bundles, store pages, franchise pages, etc.

Money talks.