Google has one single codebase. It's one gigantic repo with all of the code inside of it. Google always builds its projects from head, which means if you change an API, and lots of people use it, you need to update their code as well.
To deal with this, Google has tools for doing repository wide changes, for searching for existence of an API call, or a certain code pattern, and changing it for the better.
I've often dreamed that this would be super cool to do on Github. Send people pull requests when they are doing something wrong or they use a deprecated API.
Today, I received an issue report from an engineer at Google, which makes me think Google is heading in that direction. In this case they are changing the name of all of their Google Cloud golang APIs.