Musser and Stepanov were working together on the STL. I think they were both at SGI at that time, and introsort was integrated into the SGI STL (around the same time the STL was standardized, I guess introsort was too new to require it in C++98); from there introsort made it into most other standard library implementations which are directly derived from SGI STL.
libc++ is notable for being one of the few from-scratch implementations.
libc++ is notable for being one of the few from-scratch implementations.