There is no longer a shuttle from SouthPoint to/from the Strip. They do still run an airport shuttle.
The South Point Hotel, Casino and Spa is located far enough from the action of the Strip that if you are staying there or want to visit it for good video poker games and a meal and Steak ‘n Shake you will either want to have a car or take an RTC bus, but that requires taking multiple buses before actually getting to the Strip. Lyft, Uber and taxis are also an option, but that is going to get expensive quickly.
Visit the South Point’s Strip Shuttle page for more details.
For information on other Strip Shuttles visit my Getting Around Las Vegas: Free Shuttles to and from the Strip entry.