Who?

Ask the GP. I was just clarifying the misunderstanding. However, an extremely cursory search turns up: https://en.wikipedia.org/wiki/Slavery_in_ancient_Greece#View... where it is mentioned that https://en.wikipedia.org/wiki/Alcidamas is one such person. Though it seems that slavery in ancient Greece was normalized and deeply intertwined within their society. I am sure there were critics but like everything unpopular, may have been discarded or suppressed.

I think the question of whether any ancient society was (as a whole) opposed to slavery is a valid one.