There are some commonly used programming algorithms and snippets that have been in use and unchanged from their original C code since the 70s and 80s, because they do exactly the thing they are supposed to do, and nobody has come up with a better way of doing it. I have a fast hash function in a program of mine that was written by a guy in the early 2000s who was benchmarking various existing hash algorithms of the time, and that same function is still used in hundreds of other pieces of software.

I don’t know of entire full programs that are like this though.