Does any modern Buddhist sangha still support slavery?
Following are some of the requirements for ordination which are relevant to this question.


Must never have committed any grievous crimes. 
Should be of good reputation.
Should not be in debt.
Not subject to government service.
Having permission from parents or guardian.


So prostitutes may not be accepted as they are not of good reputation. 
Immigrant workers who are unable to find other work may not be accepted, if they are just looking for a means of getting free food and easy life. People who are in debt won't be accepted as per conditions mentioned above. A deserter may not be accepted as he can be seen as still subjected to government service or someone who is evading the law of the country. But he might be accepted in a different country. A child will not be accepted without the consent of parents. A wife may not be accepted if she is still legally married to someone and does not have his permission. 

Buddhist society recognises only four groups. Monks, nuns, laymen and laywomen. Lay people can never be slaves to the Sangha as whatever support they provide is alway voluntary. Also, the Sangha would not recommend slavery, even if it's an accepted practice in the society. Ex: Prostitution is an accepted practice in some countries. But not ordaining a prostitute doesn't mean that the Sangha recommends prostitution.