Only illegal and "outright trolling" games to be left off the service.
https://arstechnica.com/gaming/2018/06/valve-we-want-to-allow-everything-on-steam-including-something-that-you-hate/
* removed various "ero" games * Who gets to be on the Steam store? * Apple removing guns in iOS App Store promo screenshots * App Store * Google Play * Valve bans developer from Steam after it sues customers over bad reviews * an apparent disconnect