For those debating the exact geographic definition of the Deep South, there really isn't one. For me, one strong indicator would be whether an area voted for George Wallace in 1968, when the Dixiecrats left the Democratic Party after the passage of the Civil Rights Acts of 1964 and 1968 (see election results below). While not perfect, I think it captures some of the geographic vagaries fairly well. I will say, however, this particular indicator breaks down for South Carolina, for whatever reason. That's pretty surprising, considering they were the first to secede.
Today, I'd say it still remains largely accurate, except for some of the bigger cities that decided to embrace economic and social progress during the early boom in migration from the North to the "New South," which is basically what drives the economy of what we call the "Sun Belt" nowadays: DFW, Austin, Houston, New Orleans, Nashville, Atlanta, the Research Triangle, etc.