Sounds very theoretical, my experience working on some 40 year old software full of business logic, where customer A got some feature but customer B needs it to work slightly different. Aka something approaching spaghetti.

Regarding old comments I have several times used ~15 year old comments by the original author, close to the actual code to piece together the use of that code, and if I can add my fix there.

In this setting You write comments for yourself, when you in two years need to fix a bug in the new code caused by your old code. And for the next developer that will look at your code decades after you left the company.

Sometimes you, against good practice, comment out a section, with a note why, because you know this will have to be re-enabled in a few months.

Report from the frontlines...