No. Slavery is NEVER justifiable.
Evolution is a wonderful thing. Point being that human beings have revolved/progressed since our very existence came to be. With such (evolution/progression), the human species has, despite all its flaws, gradually improved in its basic existence. Particularly so in the last 300 years.
JMHO, but in order for our species to continuously proceed and excel towards overall and advanced improvement for its ENTIRE SELF, then again IMHO, the most basic concept of "forgive and forget" must be accepted and implemented.
Point being that if it were possible to go back in time and change things for the better in regards to ignorance in general, then such could and should take place. But such isn't possible.
Further, there are those who in no shape nor form to whom slavery can be traced (their families didn't own slaves, or their families weren't slaves).
It just seems to me that ALL HUMANS should focus on learning from the past - not so much on punishing others for it.
JMHOFWIW