How many people in the OOS community or elsewhere did you meet that were in favor of enslaving people. Sometimes it is necessary to ask naive questions to determine if you dig yourself into something.

those re/formed within last 100 years.

(and those who actively deny its existence /s)

You are right. It also depends if you include serfdom in slavery. In any case, just to generalize, the countries that practiced slavery were mostly those that colonized the world the most. Many of those who didn't colonize the world used serfs, like most of Eastern Europe. For these people there is no cultural association between slavery and the present. Also the proportions are different: in most countries the non-serfs were a relatively small minority. So waking up one day and reading that, say, all references to slavery are banned seems somewhat strange in other cultures.