My personal feeling is that the current top level structure (the categories listed as Category:Categories) is long overdue for a revamp/restructure. While that discussion will happen separate to this one, they are connected and influences my comment here:
Should Category:Slavery be a top level category?
My response would be No. By placing this at the head of the hierarchy in this manner, this opens the doors for other categories to become top level when they can easily be listed as equals (given the same level of importance) as other similar categories.
For instance, Art and Sports are simple topics that currently listed under Category:Categories, which gives them precedence (higher ranking) over similar topics such as Aristocracy and Nobility, Notables, and - all of which are much more thoroughly documented, nested, and used than Arts and Sports combined.
With that said, I agree with other comments that Society seems to be the current best fit for Slavery. While "Society" may not be the best word to use here (and is due for a rename), it already contains similar topical categories, such as Exiles, Conflict, Government, First Peoples, and Notables.