Inexplicable Slavery Plot?

This happened a year ago but it's still kind of appalling. For reference, this was in DnD group formed though my university's ttrpg club, so I didn't know any members of this group beforehand.

DM specifically asks us after our first session to write out backstory on our character's hometowns and send it to him. I base mine off of Alexandria with more of an ancient Egyptian aesthetic flair to it. I described it as a hidden/closed off city dedicated entirely to knowledge and scholarship. They closed themselves to the outside world long ago to protect the secrets they discovered through their studies. You might be able to get out, but you certainly can't get back in. Other than that I left it pretty vague, and added a "feel free to make some changes!" line to give the DM more flexibility and creative control.

Personally I thought you could do some cool things with this concept, like a thieves guild staging a massive heist of a magic artifact, or a political plot where the city is running out of food and resources.

DM reveals that we will be journeying to each of our hometowns, and mine is up first. We show up, and immediately see a trading port with ships of enslaved peoples of different races leaving the dock. I assume (and voice in-character) something must have changed in leadership since my character had been gone, as there probably wouldn't even be trade ports to begin with.

We investigate the entrance. DM explains that the city runs economically on slave trading, and the entire system is built around obedience and forcing labor to avoid enslavement. They specifically enslave people of other races (IE they LEAVE to go to FOREIGN LANDS to ENSLAVE people). DM treats this as the way the city has always run and not a recent development. When I mentioned that the city doesn't really interact with the outside world, he shrugs and says, "Guess I missed that part, well, a city needs to boost its economy somehow." You missed the entire description of the city? That you asked for?

I feel like it's important to mention here that I'm the only brown person in the group (brown woman, at that) and DM is a white guy. He did not establish any sort of content boundaries before this. He retroactively made my character okay with slavery and fucked up her whole backstory. It just made me super uncomfortable.

He also made the story a very stereotypical "conniving vizier tries to dupe the sultan" sort of storyline, which I felt was orientalist in an incredibly boring way. Whatever, I rolled with it, but the group just kind of naturally fell apart after that.

I feel like that's just a shitty thing to do to a POC and I've never had a DM so blatantly ignore the backstory I set up. He also AI generated images of his campaign settings, so that's another thing.