A GitHub link is straightforward.
Yes, the landing page for every SRFI already includes that. I'd be happy to include that in the SRFI template, too, so that the document contained a link as well.
Shell scripts and lists of implementations belong in the repo, because they change over time.
We used to provide a tarball, but it was impossible to keep up to date (indeed, many of the repos for old SRFIs started life as tarballs). Stick to the repo.
Actually, I expanded all the human-generated tarballs in place long ago, then deleted the tarballs themselves.
Also, an automatically generated "srfi-NNN.tgz" tarball is served for every SRFI. Not every author links to it, probably because I haven't advertised its presence.
Finally, a tarball containing all the SRFIs, including both documents and code, as well as "srfi-common/", is available at
https://srfi.schemers.org/srfi.tgz. That appears on the home page, and is automatically updated any time any change is made to the site.