I've never heard of anything like that, though I mostly study the antebellum so I may have missed it. Someone might have made such an argument in passing, but I don't know that it was ever on the table politically. Slavery was legal, with all the horrific bells and whistles, so as you note what they did wasn't a crime at the time and punishment would be ex post facto. There was a strong movement for punishing people who willingly aided the rebellion on the grounds that treason is very much not legal. That was intensely popular among southern unionists, who had suffered considerably during the war and were often the political rivals of the planter class beforehand. It wasn't their only interest come Reconstruction, but it was probably their signature one. This resulted in various laws prohibiting former rebels from voting or holding office, but with the exception of the disqualification provision in the fourteenth amendment most of those didn't last very long.

But to get back to your question, even if we set aside the constitutional problem there are some other things in the way of prosecutions for slavery-era offenses. The first is that right after the war, when the iron would have been hottest for this kind of thing, southern governments are still dominated by whites and have most of their prewar laws still in force. So black people can't testify in court (sometimes at all, sometimes just against whites) and are barred from juries. Getting a white jury to convict a white man of a serious crime against a black man isn't impossible, but it would be a hard job of work even in more racially egalitarian states. It would be pretty unusual for such a jury to punish the guilty on par with what they would do if the victim were white too.

Let's set that aside too. The southern state governments have the police power in their jurisdictions, but they're not the only players in the postwar South. You also have the national government. It's dominated to an incredible degree by Republicans, exactly the people you would think want some justice done. If not for the slaves, and it mostly wouldn't be, then they'd want it for the war that the South just put the nation through. But this never amounts to much because the national GOP is very keen on establishing a Republican party in the South. They think they can pick up a lot of the old school southern Whigs, many of whom shared a party with future Republicans into the early 1850s.

This is a little venal of them, but the GOP faces some pretty serious constraints on its freedom to act. The freedpeople don't constitute a majority anywhere in the south except South Carolina, Mississippi, and Louisiana. Even if they had total political equality and freedom, which the never quite managed, and could act politically without violent reprisals (no chance, the signature violence of Reconstruction is specifically against black political agency and its white allies) there just aren't enough freedpeople to build a coalition on. They need white allies from somewhere and neither immigration from abroad nor northerners moving South is anywhere near sufficient. Local southerners are a vital part of any successful GOP coalition in the section. The pickings here are slim, largely confined to southern Unionists. The Unionists occasionally do take up the freedman's cause, but for the most part they're indifferent to actively hostile. Many believe, like their most famous member Andrew Johnson, that the former slaves will coalition with their former owners to keep the poor whites from advancing.

We could be idealistic and say that right makes might, but there's a reason the saying holds the opposite. Absent a standing political establishment devoted to black rights, freedom would soon turn into a farce. That was precisely what the postwar South initially tried to accomplish, preserving every bit of slavery they could. You need party machinery to fight that, or you need the kind of massive, open-ended federal intervention that the Republicans were for the most part (the Radicals being the notable exception) unwilling to contemplate.

The Republican approach to black rights is critically flawed from the get-go and sometimes runs very similarly to the planter imperatives: the cotton needs growing and blacks have to grow it. So you see the Freedmen's Bureau trying to force freedpeople into yearlong labor contracts that aren't quite slavery but look a whole lot like it. You'd be stuck on the plantation all year before you get paid. You would not be free to move around off the premises, entertain visitors, and so forth. Your day to day life would remain under intense white supervision. The planters even literally write into them that all things will be as they were in slavery. The black codes, which put the state in charge of a similar situation, develop in tandem with this but don't endure long.

The Republicans want the contract regime because they think the freedpeople need it (but whites do not) and they see cotton as vital to getting the South back on its economic feet. That's part of a general program to economically develop and modernize the section, which they view as fundamentally backward. The end goal, for the more idealistic, is a South utterly remade in the Northern image with at least something like the prewar northern village capitalism and probably the wartime and postwar big business developments. Modernization is supposed to fix everything in itself. That would break the power of the planter class, provide the blessings of free labor to the freedmen, etc. But this stuff costs money and most of it is not coming from the United States Treasury. Nor can it come from impoverished Southern governments. It's got to come from outside, private investment.

Investors don't like political chaos all that much, by and large. They want a stable, secure place to invest so they can make big money instead of just throwing it away. Stability means a stable labor supply that produces reliable profits. It does not mean at all a labor force in transition, highly mobile, or inclined to do anything that would reduce profits like negotiating for better wages, asserting political independence, or demanding government action which will raise taxes and otherwise complicate their lives. Appealing to their interests to reconstruct the South was done to a huge degree, most conspicuously with railroad promotions, but the freedpeople aren't going to stop wanting freedom and the South is still pretty wrecked from the war. There are much better investment opportunities in the American West for pioneer and development minded capitalists.

And that brings us to another reason that we don't see any kind of punishment for slavery as such: the freedpeople aren't that interested themselves. They're not happy about the past by any stretch and are actively committed to creating a new world for themselves, but one of the biggest avenues they see for that is land reform. There's a lot of land abandoned or confiscated for nonpayment of taxes and/or because its owners were rebels. Why not parcel it out and give it to the people who worked it? They created the fortunes that bought and ran plantations. Why shouldn't they have it? There are some early and fleeting gestures toward this, most famously in the South Carolina and Georgia lowcountry, but in the white political world this is seen as almost unspeakably radical. Even provisions to put the land up for sale at discount rates, rather than hold it in the expectation that taxes would be paid or it would be restored to rebels, is pretty far out there to them. They wanted to deprive a white man of his land, the source of his independence, in order to give it to blacks who would probably use it for some backward nonsense like feeding their families? So they could opt out of the free labor system in favor of subsistence? That was going decidedly backward, and that blacks wanted such a thing was seen as proof that slavery had degenerated their minds and morals to the point where they required a white man's instruction. And even if they were on board with that, were the wartime confiscations really constitutional?

All of this, and more, adds up to an environment where any kind of retribution for slavery would be incredibly difficult even if no constitutional bars stood in the way.

Sources

Foner's Reconstruction