Debian/Ubuntu source archive out-of-date/not populated

Whilst debugging Virtualmin/Lets-Encrypt/DNSSEC issues (mentioned in another thread) on Ubuntu 18.04 I needed to search the list of files installed by the packages there-in so added an entry to the APT sources file and tried to update the lists but hit an error:

cat /etc/apt/sources.list.d/virtualmin.list

deb virtualmin-universal main
deb-src virtualmin-universal main

apt-file update

W: Skipping acquire of configured file ‘main/source/Sources’ as repository ‘ virtualmin-universal InRelease’ does not seem to provide it (sources.list entry misspelt?)

Looking at the directory with the browser shows the Sources.* files there haven’t been updated since 2010 and there is no GPG signature inline (InRelease) or external (.gpg).

[DIR] source/ 2019-08-18 02:48 - [ ] Sources.bz2 2010-03-09 08:14 14 [ ] Sources.gz 2010-03-09 08:14 20

It would be good to ensure the matching source-code and supporting package lists for the archive is easily available in this way.