I've heard it claimed that Tibet prior to annexation to the PRC and socialist economic reforms was a Theocracy that had a feudal slave based system, to what extent is this true?
While you wait for fresh answers from qualified experts, you may be interested in some past threads about the topic.
There has been some claim that the Dalai Lama presided over a feudalistic/slave Tibet until Chinese Communism abolished the system. How accurate is this? with a thorough response by /u/JimeDorje.
What was the system of government in pre-1950 Tibet?, also with a detailed response by /u/JimeDorje.