Obviously Fortran is used a lot in HPC and numerical routines, but I always figured those were closed-source and rarely updated, with development focusing on their wrappers.
There are quite a few that are open-source, and often under very permissible licenses, even public domain, because they were developed under university research programs.
I remember finding at least half a dozen, if not more, open high-performance libraries that implemented a direct method for solving sparse linear systems. I'm fairly sure that applies to most everything else.
Perhaps even just the same commits replicated in lots of people's forks of (say) scipy -- do they control for this? what about rebases of the same commit?