Tibetan monasteries as they operated historically are an artifact of history. Further historical and archaeological studies would be the only way to gather verifiable data, facts, figures, and artifacts from the period when the supposed slavery occurred.
This conversation seems like it is mired in historical revisionism on all sides. I’m not a Chinese Tibetan anthropologist or archaeologist, nor am I a Tibetan Buddhist scholar. I don’t know the facts of what happened, or how their society interpreted those events and the people who experienced them. For me to stand in judgement of such a nebulous concept would be irresponsible.
That you are convinced of the correctness of your beliefs is good; you should seek to share your sources so we too can be similarly convinced. Otherwise we’ll just have to take your word for it, which is no better a place than we’re in now.
For what it’s worth, slavery is much worse than being a serf. However, being a serf was likely so miserable that the distinction to our modern sensibilities may be one without a difference. I am opposed to forced servitude in all the world in all its forms.