The Lost Cause vilified southern usionists as scalawags and traitors.
Unionism was much more prevalent in the Appalachian highlands, where there were fewer slaves, than in the planter dominated lowlands. I have always suspected that this had something to do with the later cultural vilification of the Appalachian hill people as ignorant hillbillies and inbreds.

Until well into the late 20th century, southern state governments treated the highland areas, the hotbeds of unionism, as though they didn’t exist, with systematic denial of public services, education, and legal protection. This was also true in the border state of Kentucky, where Confederate sympathizers gained power after Reconstruction and set up a regime that was more Secesh than the actual secessionists in many ways.

In the 1960s, when LBJ’s Great Society came along, the rest of the country became aware of the debased condition of “Appalachia,” and it became a national scandal. The eastern counties of Tennessee and Kentucky are still among the poorest and most backward areas in the country or, indeed, in any industrialized country.
There were other causes, of course, but I would welcome a study of the role of Lost Cause prejudice in the decline and impoverishment of southern Appalachia.