That's why when I modify a free software package for my own personal use and gratification, I'm not forced to distribute the changes to anyone, not even upstream.
Archive.org's copies, like the one at https://web.archive.org/web/20190212143902/https://wagtail.i... , seems usable. Plus, things like http://docs.wagtail.io/en/v2.4/ are still live.