Hi! You’ve hit on a question which historians have spilled quite a bit of ink writing about over the past few centuries, and one on which the consensus has changed quite a bit over that time.

However, the short answer is that Roman slavery was flourishing in late antiquity and only receded (in the west) when the empire itself collapsed (though slavery did continue past the end of the western Roman state, it was transformed and diminished by economic changes that accompanied that end, to the extent that it eventually no longer resembled Roman slavery). Slavery also persisted in the Eastern Roman Empire with its capital at Constantinople after the collapse of the western empire.

Kyle Harper’s 2011 book Slavery in the Late Roman World AD 275-425 is the book that commands this field and heavily informs my answer here. As I said, though, the academic consensus on Roman slavery has changed over time, and if you look at older scholarship from decades past (or pop/Internet history that relies on outdated sources, which happens a lot), you may find dramatically different answers. Over the years all sorts of endings have been proposed for Roman slavery – that after conversion, the new morality of Christianity eventually stamped out the slave system, that thanks to imperial edicts slavery effectively mutated into proto-serfdom in late antiquity, that the Romans basically slowly ran out of slaves after the empire stopped expanding and their wars of conquest stopped bringing new slaves into the system, and others – but are not supportable in light of modern scholarship.

One reason for the large amount of revision of old theories that’s taken place in recent decades is the role that Late Antiquity has played in historical narratives, as a transition between classical antiquity and the Middle Ages rather than an era in its own right. Especially in older scholarship, Late Antiquity is often not really the period of interest. You’re either a classicist whose attention is on the earlier, and perceived to be far more glorious periods of classical Greece, republican Rome, and the imperial zenith of the Pax Romana, or else you’re a medievalist interested in tracing the roots of medieval Europe, using Late Antiquity as a sort of overflow space, a conveniently available screen onto which you can project back the origin or project forward the future of features of the era you’re actually interested in.

And if we dug into some of the no-longer-supportable models of late antique slavery I mentioned above, we’d find that that’s exactly what is going on.

Take one, for example – the Romans basically slowly ran out of slaves after the empire stopped expanding and their wars of conquest stopped bringing new slaves into the system. This theory, the conquest model in which the rise and fall of conquest-driven supply of slaves produces a corresponding rise and fall of the slave system, isn’t mainly rooted in late-antique evidence. Rather, the evidence that it latched on to is in literary sources from many centuries earlier, specifically, Roman histories covering the aftermath of the Second Punic War which state that it was after this period of conquest that slave labor began to displace free farmers in the Italian countryside. We’re thus using middle-Republican evidence to explain a period four or five hundred years away!

Or if you’re a medievalist looking for the roots of serfdom, hey, maybe we can use Roman slavery to explain that! All those slaves have to have gone somewhere, right, why not use them to originate our serfs?

In both cases, the models of late antique slavery were not being rooted in evidence from late antiquity itself.

Slavery in Roman Late Antiquity

If we focus on the late-antique evidence itself, rather than using late antiquity to explain evidence from other periods, there is actually ample evidence that slavery flourished in the late empire and was not vanishing or in decline. Census records, though limited to a few surviving fragments, suggest that there were plenty of slaves to count and no particular supply shortage. Slave sale records, though not copied and preserved in later centuries like literary works might be, have still been found in places where dry climate, local circumstance, and luck have allowed originals to survive. Allusions to slavery and remarks about slaves remain pervasive in letters, sermons, and various other parts of the literary record (Among others, Harper spends a good deal of time with John Chrysostom, the archbishop of Constantinople in the late fourth and early fifth century). Speaking of which, many newly Christian Romans seem to be quite happy to keep on with the slave society bequeathed to them from their pagan ancestors.

Collapse in the West

So in this picture, slavery is not already on some historically inevitable path of decline in Roman late antiquity. Evidence doesn’t show us a late Roman Empire that is running out of slaves due to the lack of wars of conquest, or one in which slavers are setting out to end slavery thanks to the pangs of their newly Christian consciences, or one in which there sort of just aren’t many slaves anymore but there are a whole lot of new proto-serfs taking their places.

Instead, when the western empire went down, it took (western) Roman slavery with it – with demand for the products of Roman slavery declining as instability and political fragmentation that followed the Empire interrupted the market-oriented trade that Roman agricultural slavery had produced its commodities for and erased or shrank the cities that Roman household slavery had existed in.

Ultimately, Roman slavery thrived as long as the empire did and receded along with the long, slow recession of western Roman society that followed the end of the western empire.

What Remained After the Collapse – East and West

The end of the enormous Roman slave system that had stretched across the Mediterranean and western European world did not mean the end of slavery in those regions. Littler slaveries persisted in the smaller and less interconnected societies that succeeded the empire in the West. Localized and smaller-scale slave labor persisted, as did slave trading, now often for export. Often, these post-Roman societies exported slaves to the still stable and wealthy markets of the eastern Roman world, which were still able to sustain high demand for the products of slave labor – and thus for slaves. A slaver of the centuries after the collapse of western Rome might well load his human cargo at Marseille, Carthage, or later, Venice, and set out, no longer for the diminished markets of the post-Roman west, but for the Roman and later the Islamic East.

/u/textandtrowel has previously answered What happened to the Roman system of slavery after the fall of the Roman Empire? Was the legal basis of 18th-19th century slavery derived from Roman law, or a completely separate system? but others may have more to say on the matter.