[teilo]

In all likelihood, these are NOT the user's private SSH keys, but a key generated for the service that the user is developing - such as a key to run backups and issue remote commands. Thus I can well see that the key could be committed as part of an application stack.

[mkhattab]

No, some of these repositories are user's dotfiles. See this for example, https://github.com/gomachan/dotfiles

There are many examples like this.