The rear electric sun shade was installed since beginning of production. It was also installed on the previous S-class (W126), although the option was not as popular then as it became popular on the W140.
In some markets it was standard, but very few.
It usually was standard only on the S 600, an option for the rest of the line.
The were also available rear electric curtains for the W140 (although, they were popular only on Asian countries).
If you ordered the rear electric individual bucket seats, an additional control switch to control the rear sunshade was added between the rear seats.