I have no idea - I'm not familiar with Thamnophis circles.............but I can tell you from experience with other species - cribos for instance, that common sense ideas such as preserving locality variants appears to not be on anyone's radar.
There are areas in north western SC county and SM where you can find them. A number of years ago we were paving for Caltrans near Pescadero and saw large numbers of them, especially in brush adjacent to wetland ponds.
There were enough of them that biologists were required to come in and relocate them in areas where we were staging - they would readily congregate under trench shoring, form wood, etc.
I have never understood why a species that is relatively easy to breed in captivity is not being considered for a captive breeding program. A lot of other native species on the critically endangered list have prohibitive husbandry requirements and are hard to breed - red legged frogs, gobies and birds come to mind.
But this isn't the case with SF Garters. While the vaunted European population is suffering from inbreeding and stagnant bloodlines, a captive breeding program here in the states could revitalize the species, make them available to people here, and ease the poaching stresses on the wild population.
Within a decade these would be just as affordable as any other garter..............
Apparently, the government knows best.