Not as such, it was an explanation I had heard from someone very interested in archaeology and history, whom I trust. Take it with a grain of salt or disbelieve it until someone can present evidence (no particular article shows up on a Google search).

Still, some little clue (note that Spanish mining was a huge business at this time, and would likely have demanded slaves apart from freemen workers).

Posting this here as well, since the topic is up on Round Table of Bretonnia:

Dark Elves have ready access to Human Slaves from Bretonnia, the Empire, Tilea, Eastalia, Araby, Southlands and the Chaos Wastes on their eastern seaboard. Traversing Naggaroth's underground water tunnels and caves, they are free to strike at Nippon, Cathay, Ind and the Hinterlands of Kuresh with even lesser risk of running into High Elf patrols (see "Slaughter Across the World" in 7th edition Dark Elf army book).

Bretonnian peasants and other civilized Human commoners such as Cathayans, Estalians and sedentary Arabyans likely make perfect slaves. They are used to hardship, hard labour and obedience, and won't be as rowdy and proudly independent as the Chaos Marauder tribesmen of Norsca and the Chaos Wastes.

Dark Elves are probably the best situated slavers in the entire Warhammer world, followed by Skaven. Water is a highway, a connector.

Chaos Dwarfs with their Hobgoblin henchmen are more landlocked (still with access to two seas thanks to River Ruin and their naval tunnel to Uzkulak) and are much more confined to local slave sources: Greenskins of all types including Gnoblars (they can even break Orcs!), Ogres and northern Marauders, with distant realms requiring expeditions of some expense and size to reach (being a main motive for long expeditions with mechanized war caravans). They are still great slavers outside of the Dark Lands, and one would expect many Marauder tribes to trade slave captives and other valuables for wargear forged in the Dark Empire of the Blacksmiths of Chaos.

Araby, the pirates of Sartosa and especially Norsca are avid slavers as well, with good contacts in many ports around the world. Skaven are a scourge virtually worldwide.

As to Dark Elves, one would expect them to use some slave galleys even though their Man of War models and other artwork don't sport any. Sacrifices and sport are one thing, but most importantly they run the unskilled parts of the economy by heavy labour not least through mining and farming. In the 7th or 8th edition Dark Elf army book there is a mention of a mine with Warpstone amid its ore, where dead slaves are resurrected as Zombies yet still whipped into continuing their work by their Druchii taskmasters.

Also, the Dark Elves use some slaves as servants, and one would expect that excess slaves as trading capital would be highly useful to secure temporary alliances with Skaven, Chaos tribes and Chaos Dwarfs.

As to Slann in older editions: