Assuming I've understood correctly what subscription notifications are (you subscribe to a thread and get notifications on updates), I kinda feel like for some games, that wouldn't be an advantage, especially if the OoC is a bit full of natter and idle, off-topic chitchat - a huge wedge of notifications doesn't help identify which posts are of importance.

Another possible solution might be, when we get custom extra tabs in threads, the GM can reserve one for announcements and manage a separate subscription system for that.