I've once heard they cancelled plans when the Southern Expressway was announced because that removed the need, but the majority of people that would live near this road would get off the Expressway at the Panalatinga Road exit anyway, as Lonsdale and Noarlunga are so far west of States Road.
They've got more than enough width in the corridor, so no property acquisition would be needed, and the land is extremely flat, so it would be extremely cheap for a kilometer of road.
Google Images already recognises it as a road

I've made a few images of the affected area in Photoshop over the last year, thought I might share them here.
Pana's & Cox's Hill Road Realignment.

Taylors & Pana's Intersection

States/Doctors/Pana's Roundabout.

My street directory from 1979 also shows that the road extends all the way down to Penneys Hill Road (now Upper Penneys Hill Road) to the South. The former road now services local houses, then where the map says "Road", has no evidence of a road, and it way too steep, so may have just been a horse track. Then the following section is now Kimbley Road which joins to UPH-Road.

Anyone else who would like a Panalatinga Road extension, please like my Facebook page?
https://www.facebook.com/ExtendPanalatingaRoad