The number given in that article is 27 million slaves. Yet Wikipedia claims that 55 million people lived in the Roman Empire in AD 300-400. Were less than half of them slaves? (And that's ignoring the slaves in the rest of the world at the time.) The same page claims that in 1750, the world population was almost 800 million. Were 29 out of 30 people at the time really free? Surely slavery was more widespread among the hierarchical city cultures that left written records than among the "barbarians", but it's hard to imagine that the number has always been less than 27 million. During the Middle Ages, throughout all of Europe, the vast majority of the people were serfs, bound to their land, living and dying at the mercy of their lords.

If it's true that 249 out of every 250 people today is free, that sounds like a huge improvement over almost all of human history.

Moral standards in general can improve irregardless of the number of people involved. Besides, one could argue that having more slaves is outweighed by having much more non-slaves living good lives. In regard to cryonics: all else being equal I'd favor to be reanimated in a world with low slave to non-slave ratios if I preferred the probability of my becoming a slave to be as low as possible.