This is why every major project I work on has a ".archive/oneshot/" folder for anything complicated that was used to quickly put out a fire. Not only will I inevitably need to remember months later what happened, but I'll also want to reuse certain bits later.

That ".archive" folder itself is then a personal git repo. Sometimes you just have to put that fire out or get that question answered and its not worth a "proper" solution, but keeping a history is worth it.

Resist the urge to keep it as part of the main project's repo at all costs. It's far too easy for it to be come standardized (e.g. by other devs, perhaps) and breath life into them they should never have.