Straight up one of the most extreme hit or miss systems Arelith has. A hit is so extremely rare and basically gets you good extended prisoner rp (already possible) and a miss is wildly immersion breaking half-naked-lap-elves-sexy elven fun times-in-Hub slaves-as-basically-underdark-[surface race]-buddy stuff, and we all know which one happens more often.
Ignoring the weirdness of having chattel slavery ingame, and ignoring the weirdness of unbreakable mega-collars on characters that fight demons-
It's also exploitable. Anyone that plays in the UD for long enough and pays any real attention can fairly easily compile a list of slaves that grind up, buy their freedom, and then function as a mega-outcast that can use the Hub portal and also go on the surface with no outcast tag.
So, beyond just removing them, some ideas. One, some all:

Upkeep Cost. This incentivizes a more rp-friendly Mamluk-esque system, where a slave must be actually maintained and, presumably, pull their weight. (this also prevents a character having, say, hordes of gnome slaves giggling in the UD like it's funny to be enslaved)

Level limit. This would, frankly, nip a good deal of it in the bud. Is it fun to be a level 30 wild elf monk in the UD, especially given how monks are? I mean, I guess. It'd be less destructive to the setting (sorry for the setting enforcement stuff) to be a perma-slave with no real rp goals if you were capped until freed, however. Stops the 'oh my slave is an archmage nbd' stuff, as well.

Freed slaves have to choose between becoming an Outcast (With tag, boat problems, etc) and finding their freedom on the surface (no Hub portal, no outcast tag, probably forget UD portals as well). This prevents mega-outcasts and the extreme weirdness of a slave deciding "Oh, this place I was horribly tortured and treated as an object? I'll just hang out here. It's cool. What's psychological trauma?"

Just remove it. The prisoner tag does basically anything you'd legitimately want to RP with slavery and it does it with a much less exploitable system that requires significantly less OOC collaboration, which will help to nip the harem-slave-jutsu that often bogs down the system and setting.