I thought it would be worth mentioning that after doing a complete reinstall of the game and mods, the enslaved at my slaver outpost have reverted to being called "settler". I'm only able to trade with a few of them, but they do all seem to have collars on. The voices are the original settler voices. I found one I could trade with, took his collar, then replaced it and made him reequip it. He is now enslaved again, with the enslaved voice.