[Izkata]

> Sure, documentation can go stale, but even a slightly inaccurate accounting for the reason would have, at the very least, served as a clear reminder that a reason did indeed exist.

Which is borderline the reason for version control: Do a git/svn blame on that line, find what commit it was added, and see what the commit message was. Bonus points if it links to a case on a system you still use. Sure the commit message can be useless, but it's at least something you're forced to enter when committing code, rather than external documentation that can be missed and now be misleading. Version control can even show you that codebase at time that change was made so you can see it in context (which has saved me a few times, showing what something was added for so I could confirm a suspicion).