gnome-shell no longer offers to install updates on shutdown
This might not be a gnome-software issue, but let's file it here for starters to debug what has gone wrong.
I have several updates available in gnome-software, but I'm not prompted to install them by gnome-shell's power off dialog. In fact, I don't remember the last time gnome-shell prompted me to install updates when powering off. I think it's probably been broken since the release of Fedora 29, if not earlier. /var/lib/PackageKit/prepared-update
exists and looks normal.
This means that, in practice, I'll never install the updates, since I don't want to interrupt my work to reboot and install updates during the middle of the day. I want to wait to install updates when I am shutting down my computer at the end of the day.