I see that in https://www.staging.scheme.org/ there is already a list
of Scheme standards, from R5RS on.  I suppose R4RS and earlier standards
were not included because there are no actively maintained
implementation of those today?

Those of course are still relevant, and I suppose there would be a place
for them in the site. Anyway, are those available in format other than PDF?
It would be interesting to have them available in scheme.org in
machine-readable format.
For example, I'd be interested in building a cross-reference tool that
can not only give the documentation for a procedure/special form, but
also tell which standard, in which year, introduced it in (or removed
it from) Scheme.

J.