Yeah, they did. That's why people are talking about an ancient era unlock. If you're suggesting that the institution of slavery predates agriculture, then yeah, it probably does. So does archery though, and we've got that unlocking at a tech AFTER it, because of the requirements of this being a video game, and not a neolithic simulator. What on earth are you even trying to prove here?
If you put the tech on a 1st-line ancient tech then people would simply research it in the time it takes to locate a major civ to steal from. There would be 0 game effect unless it was placed on a 2nd-line tech.
Once again, what are you even talking about? A tech unlock isn't a nerf to capturing workers, just a delay. It would function exactly the same as it already does, except it would lock the ability to steal workers for basically the amount of time that it takes to get your own worker up in a normal build order.
People are citing how the existing system can be abused; your defense of the current system boils down to misconstruing what a proposed changes would even do.