That simply requires the US Federal government to help enforce any laws requiring servitude which any of the states that created the United States may have enacted. The moral responsibility for those laws rests entirely with the states which enacted them. At most, that clause in the Constitution would justify reparations to those who actually were returned to their servitude by operation of that clause.
Failing to prohibit creates absolutely no moral obligation whatsoever. For the same reason the government has no moral obligation for failing to prevent anything for which one person may sue someone else. Guilt only ever rests with those who actually do evil, never with those who simply fail to prevent it.
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.
And that creates and supports no slavery whatsoever. It establishes the same justification for reparations to the descendants of slaves that it establishes for "Indians not taxed": Absolutely zero.
There is no right to vote, and no right to be represented in Congress. That's why the citizens of China could not successfully sue over not being allowed to vote in US elections, nor over having no Congressional seats are awarded to China, for example. Voting and representatives in a legislature are privileges which can be either granted or denied as the state sees fit, without creating any cause for reparations.
In fact, the "three fifths" clause helps act against any justification for reparations, since it denies political power to those States which had slaves by denying those States (but not their Slaves, who had no voting rights) additional seats in Congress.
I suppose if I point out the federal government itself used slave labor - to construct the Capitol, for example - you would say that was the sole responsibility of the contractors.
But you make a valid point when you write, “The moral responsibility for those laws rests entirely with the states which enacted them.”
And what states were slave states? Let's name them: New Hampshire, Massachusetts, Connecticut, New York, New Jersey, Pennsylvania, Delaware, Maryland, Virgina, North Carolina, South Carolina, and Georgia.
Yes, four of the 13 slave states ended up in the southern Confederacy.
We need to own-up to the fact that slavery existed. That is the first step in rejecting the notion that the Egyptian pyramids should be torn down or removed to another location.