This...

I once worked on a massive codebase that had survived multiple acquisitions, renames and mergers over a 20 year period. By the time I left it had finally passed into the hands of a Fortune 500 global company.

You would often find code that matched an API call you required that was last updated in the mid-2000s, but there was a good chance that it was not the most recent code for that task, but still existed as it was needed for some bespoke function a single client used.

There could also be similar API calls with no documentation, and you had to pick the one that returned the data fields that you wanted.