Which period do you consider ancient? There were different eras of Rome. There was the Holy Roman Empire hundreds of years after the fall of Pagan Rome. There was the Flavian Dynasty which could potentially match up with what you're thinking of. I don't think the Imperial Cult of Rome condoned slavery under the rulership of the Caesars. It was actually very similar to America today, and the jews hated everything about it just like they hate everything about America today. That probably means slavery was outlawed if the jews hated it so much.
If you're talking about the thing with the Phrygian Cap, I have reason to believe that it was allegorical. It may have even been a sort of baptism into the energetic state of freedom, kind of like wrapping yourself in an American flag so to say. Americans are born and raised with the idea and belief ingrained into them that they are free so I think it could be similar to this. You put on the hat, and you're symbolically a free man. It could have been like an immigrant to America getting a green card.
I don't know if that's true, but I do suspect that the cap was either allegorical or symbolic, or both. In any case, I rarely trust the (((historical))) account of things in history books. Those same history books tell us that our Pagan ancestors practiced human sacrifice. I remember seeing this flattering image of my ancestors in textbooks in class that they had us study in history class:
They said this wicker man structure was a depiction of Pagan human sacrifice.
So I tend to assume a lot of stuff written about Ancient Rome and other Pagan nations is just kosher revision of history. Slavery is and has always been a jewish thing.