I like inline code comments, especially when the original commits are not well factored. But I find it too common for a discussion starting from a patch to gradually become general and ultimately involve dozens of people that had no interest in the original patch, but care a lot about the general discussion. Sometimes these involve multiple projects, in which case cross-posting to another mailing list makes sense. With GitHub Issues, you generally tag individuals rather than groups of perhaps hundreds of people (most of whom filter on their own criteria).

Later, how well does search find the 100-message design discussion starting on a commit in a PR that was later amended and then rejected? And what if you move the repository elsewhere?