A useful kind of comment we get on both questions and answers frequently goes as follows:
Related: [link description](link target)
or, less helpfully:
When this is, in fact, related material these comments are very useful to anyone interested in the question and answer. Unfortunately, our current system is to leave these sitting as comments for years, because that just feels like the natural place for them.
Comments should get deleted, where possible, and there's no real record of them once they have been deleted. I think it would be good if these sorts of helpful links, where helpful (as judged by upvote quantity and not being a joke), could be edited into the body of post they ride on as a footnote.
While nothing stops us from doing this at present, I was hoping for community approval before I start mass-editing old material as I come across it.