Actually, it was never previously codified. This is not a change so much as a codification. Previously, whether your OOC posts were counted depended on the staffer you asked. I can't remember who did what, but I personally would only count the OOC posts if the group game was just shy of 1000 IC posts, so that they would go ahead and get their forum anyway. Another staffer always counted ALL OOC posts and would give people a group forum even if they had 200 IC posts and 800 OOC posts. And still another staffer would never count the OOC posts.
So what probably happened is that staff discussed it amongst themselves and agreed that counting only IC posts was what they would all do from now on.