As usual with experimental standards that ship in the wild with "temporary" semantics, the temporary semantics become defacto standard and the "future" standard remains reserved for theoretical future use.
In
T21165 I compare half a dozen implementations and specs and observe that they all support rel="alternate" whereas only one supports (in addition to rel="alternate") also rel="edit".
Thus I'm concluding that rel="edit" is a dead standard and that at least for right now there is no benefit to MediaWiki outputting it. There is a cost for us to output it, but there is not really any signficant cost for clients to support both, and supporting both is mandatory either way for the theoretical future standard to be adopted.
If in another ten years notable clients actually supported the supposed newer standard by then, we could switch at that time.
A potential argument could also be made that it should be removed entirely. I myself have never understood why one would want a browser extension to display an Edit button outside the viewport. It seems unappealing from a UX perspective and for me personally would likely fade into
"banner blindness" and notice if it were detected and/or notice it too much if it tries to get my attention on any editable page. In any event, while I would love to hear from people who find this useful, I am /not/ proposing its removal.
-- Timo