There are already successful Steam games known to have used AI art. And this is only in the cases of the developers publicly admitting that.

High On Life used Midjourney for it's ingame posters - https://store.steampowered.com/agecheck/app/1583230/ - https://www.thegamer.com/high-on-life-ai-generated-art/

And Firmament https://store.steampowered.com/app/754890/Firmament/ - https://www.pcgamer.com/firmament-ai-generated-content/

So is Valve now going to remove these games off the store? This seems like a very terrible way to handle this - they need to make clear rules and make a public statement, not just start banning apps that they sense use AI art.