Mind Flayers in Sigil

Slavery is legal in Sigil (there's a slave market in Planescape: Torment), and presumedly you can eat your own slaves or sell their body parts to others if you want. They're your property, after all. Some societies have laws that protect slaves to some extent, and good-aligned Cagers would be pushing for those, but I think it's reasonable that in a city where fiends and mind flayers are a powerful political force, they haven't managed to do that. Chaotic good types might spend effort trying to rescue slaves (this would be illegal) and lawful good types might take collections to buy their freedom, but I'm perfectly willing to assume there are brain-eating illithids walking openly in the City of Doors.

That said, there might be different kinds of slavery, ranging from slaves bred like cattle for food to a comparatively benign indentured servitude in which a basher retains basic rights but isn't paid or allowed to quit their job. Different kinds of slaves might be branded or collared in different ways, and be worth different prices.

A more benign view of Sigil would have only debtors enslaved for limited periods of time, who aren't allowed to be eaten. I think that's perfectly reasonable, though I prefer a darker Sigil than that (obviously, I wouldn't want to live there).

What they can't do is abduct people who aren't slaves and enslave them. If the Harmonium finds out they're doing that, they'll be lucky if they're only shut down. Sigil probably does have laws restricting slavery to those who have sold themselves willingly, and debtors (who are the ones being sold in Torment) and other criminals. Not that their brains taste worse. Prisoners of war and the children of other slaves might also be legal slaves in Sigil, though this is more of a gray area and I can see freedom-loving people getting those freed.

That gives me an idea that I think is very Planescapey. What if there's a group of people - maybe along the lines of a club or even a church - who give themselves willingly to the mind flayers to eat, believing that becoming one with the mind flayers, who donate their own brains to the elder brains, is their ticket to immortality? Life as a petitioner is chancy, and your memory is lost, and some believe petitioners aren't the souls of the dead after all. But if someone can be convinced that their brains live on within the illithids, they might think apotheosis as part of the elder brain collectives is an attractive prospect.

That said, illithids eat things other than brains. They need about one sapient brain a month to survive, but they spend the rest of the month mostly eating other things - lichen, molds, tacos, whatever. They're omnivores. An illithid pub isn't necessarily just for brain sushi; it could have exotic alcohols and meals that even a vegetarian bariaur wouldn't necessarily frown at tasting. An illithid pub might only openly sell foods not made from sapient beings, and keep the brain trade under the table. I can even see an illithid running a perfectly ordinary pub with no brain sales at all.

Of course, post-Faction War, just about everything is legal in Sigil, since there's no real controlling authority short of the Lady of Pain, who doesn't care what people do as long as they don't threaten the city itself, worship her or Aoskar, or cast doubt on her authority. Anything goes there, culinarily speaking, and illithids can even abduct people off the street if they want, as long as they have the power to defend themselves from the Sons of Mercy and other groups who object to this.

On the other hand, that means the law doesn't protect them from anti-slavery groups who decide to burn their pub down; post-Faction War, the illithids would have to be very powerful to defend themselves from githyanki and githzerai outraged at their very existence. They might have to lurk underground or conceal themselves with long hoods to avoid the armed gangs of people who hate them; they don't really have any allies who would be particularly inclined to stick their necks out for them. They're probably better off with the factions in charge.