Dorian,
There has been no official policy change, if there was then we would be telling you all about it.
Its probably more about us trying to stop threads degenerating in worthless (or time consuming) threads, or to ensure that those threads pose as much value as they can in the future. But this can just be done by moderating the thread and deleting all the crap!
Also in some cases, locking stops old threads getting dragged back up months (and in some cases year or older)....
But sure it will be discussed, if it seems like this is a major change......