Comment 9 for bug 1654070

Verified with 1.1ubuntu1.18.04.7~16.04.2:

Added a not existing dep to git:
# vi /var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_xenial-security_main_binary-amd64_Packages

# apt-cache show git
...
Package: git
Architecture: amd64
Version: 1:2.7.4-0ubuntu1.6
Multi-Arch: foreign
Priority: optional
Section: vcs
Origin: Ubuntu
Maintainer: Ubuntu Developers <email address hidden>
Original-Maintainer: Gerrit Pape <email address hidden>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 23556
Provides: git-completion, git-core
Depends: libc6 (>= 2.16), libcurl3-gnutls (>= 7.16.2), libexpat1 (>= 2.0.1), libpcre3, zlib1g (>= 1:1.2.0), perl-modules, liberror-perl, git-man (>> 1:2.7.4), git-man (<< 1:2.7.4-.), zz-not-existing-dep
...
# installed old git
# apt install git=1:2.7.4-0ubuntu1
...
# unattended-upgrade --dry-run --verbose --debug
Initial blacklisted packages:
Initial whitelisted packages:
Starting unattended upgrades script
Allowed origins are: o=Ubuntu,a=xenial, o=Ubuntu,a=xenial-security, o=UbuntuESM,a=xenial
...
Checking: git ([<Origin component:'main' archive:'xenial-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
pkg git not in allowed origin
sanity check failed
pkgs that look like they should be upgraded:
Fetched 0 B in 0s (0 B/s)
fetch.run() result: 0
blacklist: []
whitelist: []
Option --dry-run given, *not* performing real actions
Packages that will be upgraded:
InstCount=0 DelCount=0 BrokenCount=0
#

Now git is not listed among packages to be installed.
The message of not being in an allowed origin is correct, git from -updates is installable. There could be a better debug message and when I add the missing dep to the version in -updates, only the "sanity check failed" is printed.

Users can turn to APT to explain:
root@x-uu-verify:~# apt install git
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 git : Depends: zz-missing-dep-updates but it is not installable
E: Unable to correct problems, you have held broken packages.