The Deep South is what used to be the Cotton Kingdom: wherever the local economy and most powerful social class depended on the raising or export of cotton sustained by intensive and extractive exploitation of slave labor. Some states that was wide-spread enough to just say they are Deep South: Louisiana, Mississippi, Alabama, Georgia, South Carolina (although not uniformly, parts of Northwest Alabama are very different for instance). It also includes chunks of Arkansas, East Texas, and Galveston/Brazoria County.
Tobacco Road in Virginia, NC, and parts of Maryland shares important similarities but important differences as the way the crop is raised and marketed— and the origins of the planters— created a different dominant values set and mentality; ditto with Appalachia and places like much of NC where yeoman farmers may have owned enslaved people but worked alongside them to raise food crops for the household.
The Pine Curtain is somewhat of a good secondary indicator. Pines moved into abandoned land either naturally or were planted industrially and thrive in poor soil that was subject to intensive cotton farming. The pre-Cotton South was covered with hardwood forests that would eventually return absent intervention. It’s all still monoculture.