well
blowing my ever so small trumpet,
A very comact piece of control and instrumentation code I wrote for part of a somewhat novel (at the tme) monitoring system for an industrial machine is still being used unchanged on the five original installs 31 years later - an output of a PhD project. Subsequently rewritten (mine was coded in 6502 assembly) for new incantations that I believe process aircraft turbine blades, it seems to be still running, remarkably. I can only presum,e that the machines were repurposed for other similar processing and being dumb and simple no changes were seen to be necessary for what are no longer safety critical systems.
From a conversation about 10 years ago, they were still running on the same hardware . Spares have been laid down.
The softwa, if not the hardware was a test stufy in what might be done in more formal verification.