The lesson? A similar lesson to slavery, the Shoah, the Trail of Tears, etc.
"It wasn't bad because it happened to Black folks or Jews or American Indian tribes. 'Never again' shouldn't mean, 'Never again exactly how it was done last time.' It was horrible because they were human beings. It was horrible because folks went out of their way to stop viewing them as human beings. And here are some reasons why people back then (and today) justified these acts, so that when similar conditions arise again—because while history does not repeat, it certainly rhymes—we can learn to recognize and hopefully prevent these atrocities before they start next time!"
That's the lesson I'd love to have taught. That no is perfect. History is the diary of a madman. Learning not just facts but also the motivations/context of historical events, so as a society we can potentially help avoid future problems and move along with less needless suffering.