How do slaves in sectors get managed?

The Sector AI will not currently (1.1.0) enslave or emancipate pops. However, the player is able to enslave / emancipate any pop, even those pops within a sector. So to answer -- the sector does not do so, and it is up to the player to assess the needs etc. of their sectors.

What the Sector AI will do is make sure that any slaves it does have are working Food / Mineral tiles, rather than Energy or Research.