I always say I write throwaway code that never dies. I shutter to think how many pieces of code I wrote 10+ years ago are still buried deep in systems running today. Shutter.
It is always the worst code you wrote that survives. There's a terrible university dorm management software I wrote eight years ago as a student. They still use it. The crazy complicated test framework wrappers for some hardware I wrote five years ago. They still use it. The godawful and crazy complicated communication protocol I whipped up four years ago, still used in medical equipment today.
The crappy scripts that I wrote while teaching myself to code at an electrical engineering / architecture firm are used more often than the professional software I've built for FAANG and Fortune 500 companies since.
Last year I had a project to upgrade the PLC of a machine to the newest generation. As usual the customer was not able to tell me the requirements they had. They told me to look in the old software...
It turns out it was 30+ years old software where you had to program in a cmd line (Siemens S5 if you know PLCs). I had to migrate everything to the next generation (S7) to be at least a little bit productive. Then I thought come on lets try to migrate to the current generation (TIA) to be even more productive.
At the end everything was nearly ready to be compiled and uploaded to the PLC. So I fixed some minor compile time issues, deleted around 50-75% of the old program (old stuff which went obsolete), changed some variable names, refactored some stuff and here we are: The same 30+ year old software is running strong. 24/7 since 6 months without issues 😁