Good catch!
Ubuntu 20.04 LTS is EOL (not supported anymore without a special continuation license). @stgraber said a few weeks ago that he is removing the Ubuntu 20.04 LTS images from images.linuxcontainers.org Therefore, the documentation on the Demo site should be updated to reflect these.
The source code of the demo server is at GitHub - lxc/incus-demo-server: The Incus demo server
However, the messages that guide you through the demo server are found at linuxcontainers.org/content/incus/try-it.html at main · lxc/linuxcontainers.org · GitHub (Enlish).
It should be an easy task that can be done through the Web interface of Github. Can you update the Ubuntu version to something newer like 24.04? Also, do this for the German, Japanese and French pages.