I can’t find any info about a blanket ban on idle/clicker games, and quite frankly, I don’t see why valve would ban them.

For them, it’s free money:

1. the developer has to pay a flat fee for the game to appear on the storefront
2. the developer has to pay an additional fee to enable community items
3. both the developer as well as valve receive a percentage cut of the sale price in the community marketplace