Do we know if slavery is, or ever has been, practiced in Lorath?

Unlikely.

On the one hand, the Lorathi settlers were Valyrians, and Valyrians were a slave society. On the other hand, the worshippers of Boash “believed that all life was sacred and eternal; that men and women were equal; that lords and peasants, rich and poor, slave and master, man and beast were all alike, all equally worthy, all creatures of god,” which suggests dissent, as does the fact that “the islands became a refuge for freedmen and escaped slaves from Valyria and its proud daughters, for the priests of the Blind God taught that every man was the equal of every other.”