It goes a lot further than that. Github quite literally becomes the hub of the projects it hosts. Without the hub the spokes will have to reconnect and many of the glue bits will be lost in the process and some of those bits are very important. A public home is a very important thing to an open source project.

Even the issue tracking?

Someone will probably point out that data can be exported, but that is frankly irrelevant given that the URLs for your project are now embedded around the entire world to a domain name you don't control (github.com), and which your goal should be to "outlive". There is tons of historical content sitting on SourceForge and Google Code that has links all over the Internet pointed at, and it is a major issue that you can't migrate and take these links with you; if github.com offered "bring your own domain" as a feature for some moderate cost, it would be much more viable as a platform for people who care about the longevity of their projects.

You can use your own domain for GH pages... which many sites do use for their documentation.

Their API exposes issue data, and there are a handful of 3rd party API-based tools designed to back up / extract it.

Also, for anybody not already aware (I didn't know this for a long time): GitHub repo wikis are themselves git repos that use Gollum

I've already written a standalone tool to do exactly this: https://github.com/josegonzalez/python-github-backup

The API lets you access all that.