Much of the source code related to Steem is stored on GitHub. This includes the main protocol implementation, the steemit dot com source, and the busy dot com source. In fact, I'd gander that most open source tools from http://steemtools.com are hosted on GitHub.

Now, since GitHub stores code using git, which is an open source distributed protocol, most the source code for most projects is stored in a redundant way. Each contributor likely has a copy, although perhaps it's a bit outdated. The problem is that GitHub provides many important tools that are not part of the git repository. This includes issues (comments and discussions) and pull requests (requests to update the codebase). These are crucial aspects of a repository's history. They include all of the peer review details.

In the short term, Steem projects don't have a ton to worry about. GitHub is not going to change overnight from this acquisition. However, I think it underscores the importance of trying to move issues and pull requests to decentralized platforms. @utopian-io has started to move some discussion and peer review of code to the Steem blockchain. But there's a long way to go until anything replaces the utility and adoption of GitHub.