It would be impossible for what we Americans now call maltreatment of people of non white races to actually BE maltreatment. It would simply mean that it’s not in favor now. There would have been no antebellum evil in America to have overcome. Nor if the sentiment returned would it be evil.
We know that liberals don't really believe that though. Otherwise they would be fine with a nation wide popular vote on issues like abortion, "gay" marriage and illegal aliens. IOWs their ethics are entirely situational.