@Wayne : gnome-software got uninstalled on my system also on 12/20 during a regular system update. It would not re-install, so I installed plasma-discover, which is not as pretty as gnome-software but it does work as a software discovery tool.
Both are dumbed down and inferior to Synaptic for displaying dependencies, versions, and changelogs. I like to see all that before updating so I can see what was fixed (and what might break after an update 
)
Some days later I was able to re-install gnome-software, but I just did it this way:
$ sudo apt install gnome-software
and it worked fine.
I’ve never heard of package pureos-desktop before, which is not installed on my system and apt says it can be safely removed:
$ apt show pureos-desktop
Package: pureos-desktop
Version: 0.9.4
Priority: optional
Section: oldlibs
Source: pureos-meta
Maintainer: PureOS Maintainers <pureos-project@lists.puri.sm>
Installed-Size: 10.2 kB
Depends: pureos-gnome
Download-Size: 3,808 B
APT-Sources: https://repo.pureos.net/pureos green/main amd64 Packages
Description: Transitional package (pureos-desktop -> pureos-gnome)
This is a transitional package and can safely be removed.
It’s a moot point at the moment, because the Purism repo is timing out!
$ sudo apt update
...
Hit:4 https://repo.pureos.net/pureos green InRelease
Err:5 http://deb.puri.sm/pureos green InRelease
Could not connect to deb.puri.sm:80 (138.201.183.165), connection timed out
Reading package lists... Done
Building dependency tree
Reading state information... Done
37 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: Failed to fetch http://deb.puri.sm/pureos/dists/green/InRelease Could not connect to deb.puri.sm:80 (138.201.183.165), connection timed out
W: Some index files failed to download. They have been ignored, or old ones used instead.
$
Cheers!