Comment 12 for bug 1978125

Verified. I hacked around the Packages files locally to simulate the situation:

0. Pinned snapd to -1 and removed it
1. Modified packages file to add Depends: snapd to an update in proposed (netplan.io), and set
   Phased-Update-Percentage: 0 on snapd

Before:

root@jammy:~# apt policy snapd
snapd:
  Installed: (none)
  Candidate: 2.55.5+22.04
  Version table:
     2.55.5+22.04 1 (phased 0%)
        500 http://archive.ubuntu.com/ubuntu jammy-updates/main amd64 Packages
     2.55.3+22.04 -1
        500 http://archive.ubuntu.com/ubuntu jammy/main amd64 Packages
root@jammy:~# apt upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following package was automatically installed and is no longer required:
  libfreetype6
Use 'apt autoremove' to remove it.
The following NEW packages will be installed:
  snapd
The following packages will be upgraded:
  apt libapt-pkg6.0 libgstreamer1.0-0 libicu70 libnetplan0 libnss3 netplan.io python3-gi
8 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
1 standard security update
Need to get 34.8 MB/37.1 MB of archives.
After this operation, 89.3 MB of additional disk space will be used.
Do you want to continue? [Y/n] ^C
root@jammy:~# apt install snapd -s
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following package was automatically installed and is no longer required:
  libfreetype6
Use 'apt autoremove' to remove it.
Suggested packages:
  zenity | kdialog
The following NEW packages will be installed:
  snapd
0 upgraded, 1 newly installed, 0 to remove and 8 not upgraded.
Inst snapd (2.55.5+22.04 Ubuntu:22.04/jammy-updates [amd64])
Conf snapd (2.55.5+22.04 Ubuntu:22.04/jammy-updates [amd64])

After upgrading apt to 2.4.6:

root@jammy:~# apt upgrade
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
  libfreetype6 squashfs-tools
Use 'apt autoremove' to remove them.
The following packages have been kept back:
  libnetplan0 netplan.io
The following packages will be upgraded:
  libgstreamer1.0-0 libicu70 libnss3 python3-gi
4 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
1 standard security update
Need to get 13.1 MB of archives.
After this operation, 2048 B of additional disk space will be used.
Do you want to continue? [Y/n]
root@jammy:~# apt install snapd
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Package snapd is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'snapd' has no installation candidate