Pretty sure they can’t close the forums, actually. I’ve never seen a Steam forum be outright disabled, despite seeing many devs/publishers who would absolutely close that down as a place of critique. I haven’t even seen delisted but viewable games close down their forums from new posts. Are you sure Valve doesn’t have an agreement with publishers that they are required to keep the forum open for their games?

The discussion I have seen from publishers is that the automated moderation on Steam is literally nonexistent from their end, and the global automation is far too lax. So they push their player base to platforms that are actually capable of dealing with problematic behavior.