Perhaps we just need to apply {{historical}} more widely?
Or maybe we need a new Header-template that's distinct from {{historical}} and {{outdated}} and {{fixme}} ?
I think tagging pages is slightly preferable to moving pages, because it's less complicated to undo, and still leaves content accessible to a basic search (just further down the results), and to different use-cases (per Gergő's comment), and doesn't get technically semi-blocked for translated pages with >500 subpages.
However, it is more labor-intensive for pages with many English sub-pages (i.e. needing to tag each individually), although I'm not sure how to evaluate that issue, without preemptively seeing the massive list of the pages that ought to be deprecated!
I'd recommend: Tag a few dozen pages, and simultaneously create a listing of pages where tagging doesn't seem sufficient, so that we can discuss more specific examples/buckets of examples.
E.g. you mentioned [[Help:Extension:WebFonts]] as a candidate for Archival, but perhaps that could be handled in the same way as [[Extension:WebFonts]] itself was (blank and tag it with {{archived extension}} )? Plus I just checked, and that page has over 500 subpages (from translations) so it would require dev assistance to move!
The other semi-related problem this all makes me think of, is the cleanup needed in
https://www.mediawiki.org/wiki/Special:PageTranslation to "discourage" any outdated pages. I'm not sure how extensive that problem or overlap is, but ideally anyone doing cleanup of one problem could also keep in mind the other.
Hope that helps!
Quiddity.