I don't know what "Red Hat model" you mean, but if you're referring to their code delivery: It's not a problem for their internal developers, because they can use git lol. You can theoretically go to the upstream projects individually and merge the code drop with their nearest branch, and THEN do git blame/diff to figure out the delta and history of some lines of code. There are probably a few upstream open-source projects out there with no public VCS, but those are rare.