I did a considerable chunk of research on Theravada within China.
You can read an interview with my reflections on that period of my life, here:https://medium.com/p/9df3f5f826e4
At the start of the 20th century, the tradition was already very weak (and war-torn) in both of its major centers of activity in Yunnan (that had, BTW, remarkably little direct contact with one-another).
The violence of several sequent wars, followed by Communism, and then followed by long-term government anxiety about controlling the Burmese border, has made matters considerably worse.
My heart breaks for many of the communities involved; you can meet people out there, speaking various profoundly different languages (by no means only "Tai") who have sincerely tried to reconstruct some kind of Buddhism from the ashes of the 20th century. However, the results are no better than heartbreaking.
If you can read Chinese (or want to discuss this with Chinese-speaking colleagues) cf.https://medium.com/p/4e7ba4f7a30e
...and you'll notice that the status of the Theravada minority within China is very much an "enframing device" in this longer essay (aimed at a Chinese audience) that is posted in both English and Chinese translation:http://a-bas-le-ciel.blogspot.tw/2014/03/canon-and-reason-complete-chinese.html
I'm sorry to say that if you read all three of those linked-to articles, you'll be among the best-informed people on the subject on the planet. Very, very few people have taken any sincere interest.