Hi Steve! Sorry I missed this reply.
Well, actually I supposed there could be an ARIA role value for "footnotes", and I think it is useful especially when it comes to the point of marking footnote section.
But still there's something missing IMHO. ARIA is mainly for accessibility and it is neither a feature supposed to have consequences on general-use UAs, nor a link-specific feature.
rel attribute, on the contrary, gives information about the nature of links.
By looking at the table for HTML5 rel values, it appears that these have both a semantic (conceptual) meaning and a functional purpose. Browsers and, in general, user agents, can expose a specific interface for some of them (see note about
<a rel="help"> and "help" cursor).
I know there's no magic in attributes, and the same result can be achieved with a
whatever attribute, once there's an agreement upon where to look at. Nonetheless, it's highly arguable that this case is less worthy than
bookmark keywords (such keyword would convey 2 additional meanings to every user agent about the link: 1 - the target is probably an anchor inside the document itself; 2 - the target's content is a supplement to the text and there could be more useful ways to show it rather than navigating the document back and forth - see Wikipedia-style footnotes).
Moreover, judging by a very quick search over Webdevdata, I saw that there's a marginal use of rel="footnote" in the wild, and I suppose this is why "footnote" is listed as "existing rel value as POSH".
What I don't understand is the meaning of "Proposals: HTML5", as it is not present in said spec, as well as the reason why it was dropped in HTML4. Has there been a revision since then?