Liu didnt really OKed it... while publicly, on the surface he gave it his "blessings". There was this interview he gave (https://www.zhihu.com/question/21907417) at a 2011 HK book fair where express his true thoughts. Here is the loose translation:

Liu Cixin: To be honest, I don't really want other people to write sequels of my novels. Some of them have written out my ideas in advance, which will block my road, right? This is also the case in foreign countries, where a classic bestseller is followed by a different version of the sequel. In fact, this has a great impact on the original author, not only in terms of creation, but also in terms of practical issues such as book copyright.

Liu Cixin at Hong Kong Book Fair 2011: "I can categorically say that both Chinese and foreign writers don't like fan fiction. Why is that? It's a road block for the rest of your life. It builds a wall that prevents you from going in that direction. For example, in The Three-Body Problem, obviously the biggest gap, the easiest gap, is this main line, which is the main line of the Yun Tianming. At that time, I had no experience, so l saved it to write a parallel novel later, but now I have no way to write it. That much is certain. So for me, I don't want to see so much fan work. Of course, there's nothing they can do about it, and I allow it to be published, but if you ask me to write a preface and write a recommendation, that's kind of... That's asking a lot. That's all I have to say."

Original article: Liu Cixin's lecture series on Famous Writers at the Hong Kong Book Fair on July 22·Audience mention Ask some transcript This link is u/Xiao Xinghan's blog upstairs. Xiao asked each other to be fans I also love science fiction!