A neat real-world example of ancient Quake code surviving to this day is visible in Valves games - the hardcoded patterns for flickering lights in Quake 1 survived into GoldSrc and then into Source and then into Source 2, most recently showing up in Half Life Alyx, 24 years on from their original appearance in Quake 1.

https://www.alanzucconi.com/2021/06/15/valve-flickering-ligh...

Basically all of the bigger systems will have been Ship-of-Theseus'd several times over by now, but little things like that can slip through the cracks.