Massive threads aren't a policy, they usually happen when the individual sub-topics aren't enough to justify a thread by itself, and I don't think that changes if there's a subforum or not (see the tech forum and the video game thread). But it doesn't have to be that way, people can create a separate thread if they want.
Usually we split off a subforum when it's painfully obvious that a topic is dominating the off topic forum with threads, so people could create more threads about political topics rather than having them mushed into a larger thread.
Uncertainty is an uncomfortable position.
But certainty is an absurd one.