When you use 'go get' to add a package to your Go project, it actually fetches the code through a Google proxy which saves a snapshot of the commit in question. Even if the original source goes away, they should have a copy of every version of the library ever fetched via their tool, and devs can continue to build existing stuff.

(If you don't want Google to see what packages you're fetching, you can also turn this off with an environment variable.)