You may be thinking of the giant, metal "S" that's part of the Superman statue in Superman II. This was designed to be a part of the film's visual design and to symbolize Superman's legacy and power. It was not meant to be a literal plastic S.
The giant metal "S" was even used in the film's poster art, becoming a recognizable element of the film's aesthetic.
It's understandable that you might remember a plastic "S," as the film was groundbreaking for its use of visual effects. However, the "S" statue was definitely made of metal.