This is according to a couple of Weibo posts from leaksters such as Kang and Duan Rui. While the letters themselves were not shown, according to the leaksters, it seems that Apple is forbidding that they make posts in which they try to frame their leaks in the form of dreams or riddles, which is what some of them have been doing, which is admittedly a rather clever way of hinting at something without explicitly saying anything.
However, as AppleInsider has speculated, there is a good chance that these legal letters are probably your standard cease and desist letters. Kang has also since warned others that might want to follow in their footsteps that Apple will be monitoring them and that future leaks will be considered an “infringement and abuse of commercial information”.
Apple has been trying to crack down on leaks as much as possible but since it would be almost impossible to have full control of their supply chains, leaks are bound to happen, it’s just a matter how big these leaks are and how frequent they are posted.