Perhaps what I wanted would be fulfilled if I can filter or sort srfis by "superseded". Would it be wrong to use keyword for it (maybe it is; it doesn't feel to fit with the same category as other keywords.)
That's an excellent idea. I will add the superseded keyword to SRFIs as appropriate. I'll announce that on srfi-announce when it's done.
Contacting authors of the old srfi does sound a lot of work. That's one thing that I feel having "superseded" flag, which can be attached without authors' approval, (But if we need to change the "status" section of the document we need author's approval after all. Hmm.)
I view the Status section as the editors' purview, so I don't need authors' permission to change that. But declaring that a SRFI has been superseded by another is a stronger statement than the dates that usually appear in the Status section. That's is why I always seek authors' permission for that — and for noting errata.
So the superseded keywords I add will still be just for SRFIs that the authors have agreed are superseded. See-also links will have to do for the rest.