I was just trying to illustrate my use case, I didn't mean to imply anything about lines you can't enter. I typically am editing source files where all lines can be entered. But now that you mention it, if for example there was an overlay making it appear as if more lines were in the buffer than there actually were, then yes ideally those would not count. That's not something I run into very often though. A solution that didn't handle this edge case but otherwise had relative visual line numbers would still be a big improvement.
From a text editor programmer's perspective line number may be defined by something like, "number of preceding newline characters" but I think most users think in terms of the lines of text as they are rendered on the display. Most non-emacs editors and IDEs nowadays use wrapping that acts like visual line mode by default. I would call them "visual line numbers."