No. It's absolute baloney and incredibly racist.

The 1860 U.S. Census found that 40 percent of the population of the South was enslaved. I'm quite sure that they were aware, every single day of their lives, of their status as a slave.

But let's take the racist's point of view. Let's say a slave isn't a person and therefore only white people matter. Even then, the assertion is dead wrong.

One-quarter of all free families in the South (note, this category includes black free families) owned slaves, according to the 1860 U.S. Census. One half of this proportion (12.5 percent of the free population) owned five or more slaves. Now, let's look deeper. In the Lower South (seceded before Fort Sumter), 36.7 percent of white families owned slaves. In the Upper South (seceded after Fort Sumter), the proportion was 25.3 percent. In the Confederate states as a whole, it was 30.8 percent. In the border states (which did not secede), the percentage of slave ownership was 15.9 percent. In two states, South Carolina and Mississippi, more than half the population was enslaved.

From Armisted Robinson's Bitter Fruits of Bondage: The demise of slavery and the collapse of the Confederacy:

Most Americans, no doubt, imagine the prewar South as a region so thickly dotted with immense plantations on which most of the black and white populations worked and lived. But, on the contrary, while slaves made up 40% of the total population of the South, only 25 percent of free families, most of them white, owned any slaves at all, and fully one-half of this minority (12.5%) held fewer than five slaves. Only an owner of twenty or more slaves, and of substantial land, could qualify as a planter, and fewer than 10 percent of slave-holding families qualified. The plantation elite of the antebellum South made up less than 3 percent of the free population in the region and less than 2 percent of the total free and slave populations combined.

Let's put this into context. You probably own stocks and bonds ─ investment documents either through a mutual fund, direct investment, your college fund or a 401(k). A slave was a big investment. On a plantation with 20 slaves, the value of those slaves (in 1860) would be greater than the value of the land and all the improvements ─ houses, barns, orchards, fields, irrigation ─ on it.

In fact, the value of a single slave was so great that in 1950, only 2 percent of Americans held stocks worth more than the 1860 value (inflation adjusted) of a single slave. Restated: More than 10 times as many Americans relied on slavery for their wealth in 1860 than relied on the stock market for their wealth 90 years later.

Another important point is that even if someone never owned any slaves, yet lived in a slave state, they still benefited enormously from slavery. Think of it like Detroit. Only a small percentage of Detroit residents ever worked for one of the big car companies. But they all benefited from them, because a large percentage of the dollars from any business came directly or indirectly from the car companies. There were restaurants that served employees; realtors that sold houses, schools the children of automotive business employees attended, etc. etc. So the Detroit economy has collapsed since the car companies ran into economic trouble and closed factories, even for people who never set foot in a factory. So in the antebellum south, a free person who lived in a town where cotton was grown while slavery was legal, might have a general store that sold to plantations, or maybe was a skilled laborer like a carpenter or blacksmith, whose customers were from cotton plantations. Etc. etc. Slavery pumped a huge amount of capital into any economy it was in.

And just to be clear: Harper Lee is putting this argument into the mouth of Atticus Finch but it was and is a common southern apologist argument for slavery.

From this about 1 in 4 southern households had slaves. If one were to apply ownership to solely the patriarch of the family, then one would say that about 5% of the south owned slaves. This argument is technically correct as the kids (and maybe the wife at the time) had no legal ownership of the slave. However, they lived in a household where there were slaves used to serve them, thus they effectively owned slaves.

The "never seen a slave" part is complete bullshit.