As said in the tons of other leak threads in Suggestions & Feedback: As a general rule, we allow leak threads.
The moderator team has no way to tell which are legitimate and which are not. We don't want to accidentally shut down a genuine leak thread, and we HAVE gotten genuine leak threads in years past.
Not only do we not want to do delete a real thread, we also don't want to give people false hope with action on a fake thread. We have also have discovered from the past experience that closing or deleting threads also leads to people reading into it overly much and thinking those leaks are real (track record so far from prior expacs: none of those aforementioned closed/deleted have been). There's a thread here recently in S&F as an example. We've also had people make threads of Reddit, etc, reading into the accidental deletion of a thread. Any action we take on these is interpreted by the community as a sign of validity.
People like to speculate. As long as people are having fun speculating and discussing in leak threads, that is a good thing. If people stopped caring and responding to them, they would probably lose their appeal to make when they got no replies and dropped off the first page, and everyone and their mother would stop making BS threads. But so far the community has suggested with their focus and action that they enjoy these.
If you see a "leak" thread that is obviously trolling or fake, report it and we'll review it and close if necessary. And by "obviously fake" I don't mean "this doesn't sound genuine" I mean things that are really, really, really blatantly made up: parodies of other threads, MS paint "screenshots," Dora the Explorer themed, whatever.