Issue and PR content are not social artifacts, they belong in the repo or at least its hosting instance. Likes and activity feed are the sort of thing that belongs in the social layer.
Issues and PR content are absolutely social artifacts, as they communicate information to others, whether opened by the repo owner or not. In fact, other people can interact with them, and are supposed to do so, which makes them even more inherently social.
The more important thing about issues & PRs is that they explain why the code is the way that it is. Knowing why the code is the way it is, is almost the entire reason we have a repository to begin with. They belong to the code, not some social graph.
Yes, code is largely about communication as well - with others as well as our future selves. That doesn’t mean it’s a social media experience. There are lots of ways to communicate that aren’t that.