Look, I read the thread, and perhaps I missed it, but has anyone ever brought forward a verified, contemporary, first-hand document stating that slaveowners were deliberately attempting to engage in selective breeding?
No.
In fact, there is not even any unverified, non-contemporaneous, after-the-fact document that claims the same thing–just random comments by people with no historical education speculating wildly about what they wish to presume happened. There are numerous diaries penned by slave owners and none of them actually mention “breeding.” From those diaries, it would appear that the slaves were generally permitted to pick their own spouses, (with the understanding that such marriages were often under threat of dissolution due to sales).
Leg length has a genetic component, but a much bigger nutritional one. Your long bones do most of their growing before puberty, and the better your nutrition as a child the longer they will end up proportionately (within the range that’s possible for your genetics).
Remember that west africans tend to be better at explosive speed while east africans tend to be better at long distance events. Obviously better training/nutrition is going to be a major benefit.
Just as a note, I’ll point out that one of the strongest of the slave kingdoms was the Kingdom of Oyo, which had an extremely well regimented army with a professional officers corps, with infantry armed with swords, spears, and later muskets they traded for with the Portugese and Dutch, as as well as both heavy cavalry armed with lances, spears, and sword, and light cavalry armed with bows.
They weren’t the technological primitives you’re making them out to be, and neither were the other slave states, which in general were advanced iron age agricultural societies with strict social class systems, complex political and religious hierarchies, clear division of labor and accumulation of wealth, and complex, developed bureaucracies.
These weren’t stone age hunter gatherers living in caves. In the 16th-18th centuries, these states were on a level equal to at least Medieval Europe, and rulers equal in political sophistication to anywhere else in the world. Most of the kingdoms didn’t have horses, because of the tsetse fly, but they did have wagons, technology, tactical knowledge, and, after they started selling slaves to the European powers, guns and cannons they bought with the money they got from the slaves.