Git is an immutable history of all work ever done on code.

Take a look here: https://github.com/torvalds/linux/commits/master

And here: https://lkml.org/

Wikipedia does a pretty good job of maintaining document and discussion history as well.

In both projects, you can pull on any random thread and trace it all the way back to its origin. And if the team (and maintainer) is doing a great job of capturing the thought process behind a decision in commit messages (as Linus does so well), then you're in really great shape.