The decision to ban Chaos;Head was probably reversed because it's a high budget VN made by a well-known company with a big fanbase, so they had enough clout to appeal and get Valve to change their mind.
I am one person with little in the way of financial power or a fanbase, so I very much doubt Valve would ever change their mind re: banning Sweetest Monster Refrain, as they have no incentive to do so.