Also, even if it is authoritatively managed in git now, the whole 20 year history certainly wasn't (since git is only 14 years old, and Google probably didn't adopt it on day one), and it's quite likely commit history wasn't converted,so it's quite possible Google couldn't easily make the whole history available when publishing it to GitHub even if they wanted to.
I assume the authoritative version is still in Google's Piper-based repo and previously was in perforce and I assume that was for a while ... so if there were interest Google's could dig deep. But I assume there are other projects where this is even more interesting. (how ranking changed over time; how storage formats for the index changed; ...)
