Why not involve libraries in the effort? They could keep a "master" copy and github.com, gitlab.org, SF.com, or whoever else comes next can host development versions.
There is a need for an organization that is willing to host established but small and unfunded projects like libtiff, libpng, zlib, etc., and give them some minimal organizational backing in the interest of continuity (so no github). Somewhat similar to, but more general than, what the Network Time Foundation is doing for ntp and related projects.
It has been on my list to integrate https://github.com/joeyh/github-backup with https://github.com/ArchiveTeam/ArchiveBot.