Been building and maintaining Unix/Linux systems since the early 1980s, Gus.
You could call this “Legacy” code dating back to 1981 when I worked on the real-time OS used to run the Australian National Animal Health Laboratory (ANAHL, renamed to AAHL because people used to pronounce it Anal, and recently renamed the Australian Centre for Disease Preparedness - ACDP - because of the outstanding work they are doing there on COVID-19. I do think they should have called it the Australian Centre for Disease Control, so we could all call it the ACDC! )
The code transitioned from M6800 assembler to C (6809->68008->68340) and then to various operating systems, including AmigaDOS, VAX VMS and Tru64 Unix, before corporate powers insisted on Windows (NT ->2000… etc). The field stuff stayed on our own hard real-time OS and dedicated hardware (where most of it still is) until the move to WinCE starting around 2007.
At this stage there is no plan to move those decades of legacy C code to C# or even touch the bulk of the code, just put emulation wrappers around it, which is MUCH easier with the more recent Linux tools than it was in the past. Despite the fact that these days your smallest processor would have ample memory and speed for the job, nobody is interested in paying for the old software to be ported.