The inequality of slavery, or of Jim Crow laws, or of indentured servitude, etc., is all considerably greater than the inequality now, even though the economic disparity now is larger.

But those levels of disparity aren't functionally useful; if you have $10M of assets vs $100M vs $1B vs $10B vs $100B, it hardly matters for your functional day-to-day happiness. Numbers say "BIG!!!", but reality says, "eh, same difference".

Now, it may be that resentment over inequality is higher--maybe it's easier to rationalize your place in society when you're labeled a serf than when you're labeled as a citizen just like that billionaire over there.

Still, I don't think the "unprecedented" inequality argument holds much water without further development. Higher inequality than in the '70s, sure.