I think people are confusing depicting something with romanticising it.
When I first saw the series, I wasnt interested because of the slavery. I thought it looked like a cool historical cowboy-themed BL and the artwork caught my attention. Slavery is awful obviously, but its also a real part of history. A story including it doesnt automatically mean the author is supporting it or that readers enjoy it.

What confuses me is how people will read stories with r*e, abuse, and all sorts of messed up things, then suddenly act like slavery being mentioned is crossing a line. Both are horrible. Both involve people being treated as less than human. Neither should be romanticised.
And honestly, not everyone reading this is here for the slavery. Thats such a weird assumption to make. Some people are interested in the historical setting, the characters, the art, or the story itself. Liking a series that contains slavery doesnt mean someone supports slavery, just like reading a murder mystery doesnt mean you support murder.

If the story ends up romanticising slavery, then yeah, criticise it. Thats completely fair. But calling everyone whos interested in reading it racist or proslavery before youve even seen how its handled is just unfair and dont help anyone.