Can't find package 'xiphos' in Software Center or with 'apt', even though it is listed in the Package Search page

The Xiphos package is not available in the Ubuntu Repositories for Focal due to 'bad timing' for a fix for an issue in Debian which addresses a build-dependency problem.

Its removal reason initially was:

Debian: #889019, scheduled for removal in Debian, b-d on removed gnome-doc-utils

This means that it was scheduled for removal in Debian, and the build-dependencies on the removed gnome-doc-utils package means that the package could not be built.

However, in June of 2020, gnome-doc-utils was removed from Debian and the build dependency removed from the packages needed to build the xiphos package, and as such the block on xiphos was no longer a concern. As such, Focal is the only supported Ubuntu release that does not ship Xiphos, as that change to remove the blocking package happened after Ubuntu release. And since this is a Universe package, nobody went back to see if it should be synced or added after the fact to -updates, however that is usually rarely done.

One can always backport Xiphos in a PPA for Focal, however it doesn't look like that's being done at this point, and PPAs are not necessarily 'trustworthy' unless you trust the person who maintains that PPA.