The abolition of slavery is probably a must for Federation membership. Equality of all sentient beings is a fundamental core of Federation ideals. So if slavery exists it'll have to be abolished otherwise no membership. Although if the society wants to keep slavery, the Federation can't do anything about it because it would be a PD violation to do so. Could make for an excellent PD adventure.
Slavery probably does still exist in some form or other...indentured servitude, debt slavery...the list is endless. Different societies view slavery differently. Some approve of it or have social distinction, like Thralls and serfs which are forms of slavery but accepted social norms.
Arise, arise, Riders of Theoden!
Fell deed awake: fire and slaughter!
Spear shall be shaken, shields be splintered,
a sword-day, a red-day, ere the sun rises!
Ride now, ride now! Ride to Gondor!
Theoden King: The Return of the King