Also, in the ordination rules ceremony slaves are excluded from the possibility of ordaining. ref. Has any modern sangha made a point of allowing current or former slaves to ordain?
Historically landed Buddhist monasteries have owned slaves. ref At least this appears to be over.
UPDATE: Ancient slavery and modern slavery look different. Modern slavery generally is tolerated, but officially illegal. Example include prostitution in Japan, where immigrant women often can't quit or flee their "employeers" because the police won't assist illegal immigrants or "guest workers" trying to do something else on a guest visa.
Also, a theme in ancient restrictions in joining the sangha was that someone who owed someone something (not necessarily full on slavery) wasn't allowed to join (e.g. need to get permission from parents and spouses). So would a modern sangha turn away a soldier who had deserted or dodged the draft?
Or even further down the spectrum of slavery-- what if someone was just heavily in debt to their boss, bank and so on?
I'm particularly interested in what the modern institutions are doing. I'd google it myself, but I'm mostly find references to how the situation was long ago.