Actually you can’t comment on lines that haven’t been changed if they’re too far from the changed lines! It’s a weird limitation of GitHubs data model.
(source: that’s one of the features that people like most about the review tool I created, CodeApprove)
Annotations can be added by "checks" anywhere, though, I believe.
I like to use that to surface cross-references (annotating anchors whose references appear in changed or context lines, and references whose anchors do).
As the other commenter pointed out, only if they're close enough to a changed line. That's what I meant by "context" (borrowing terminology from diff, where you change the amount of it with -C).
(source: that’s one of the features that people like most about the review tool I created, CodeApprove)