[bbanyc]

How about the oldest software in common use? There's probably some parts of the 1973 C rewrite of Unix still around in modern Solaris, idk how common that is. tcsh also dates back to the '70s and is also falling into disuse.

Maybe TeX, written in 1982? Though pdfTeX displaced the classic TeX DVI compiler long ago, and all the major TeX distributions translate the Pascal source code to C. I don't know whether that means it's a different program from the one Knuth wrote 40 years ago.

[xmcqdpt2]

BLAS is pretty old, with a first release in 1979. Even though modern computers likely don't use any of the old BLAS code, they still have implementations that respect the original API.

https://en.m.wikipedia.org/wiki/Basic_Linear_Algebra_Subprog...