Look at the manifest. It specifies all the snaps and snap-associated packages that would be installed.
snap:core stable 7917
snap:core18 stable 1223
snap:gnome-3-28-1804 stable/ubuntu-19.10 71
snap:gnome-calculator stable/ubuntu-19.10 501
snap:gnome-characters stable/ubuntu-19.10 317
snap:gnome-logs stable/ubuntu-19.10 81
snap:gtk-common-themes stable/ubuntu-19.10 1353
The fact that you deleted them in 19.04 shouldn't have affected what's installed in 19.10 but that doesn't seem to be the case.
In 19.04, if you run
sudo apt purge snapd, doing so would remove, in addition to
snapd, the snaps presently installed on your system. Now, if you run
sudo do-release-upgrade, neither
snapd nor any snaps are installed. Two users, here, and here, have verified this. I also checked, in a VM, that the upgrade from 19.04 to 19.10 didn't reinstall snaps or
sudo apt purge snapd was run before
sudo apt purge snapd.
Whether this situation will persist if users eventually upgrade from 19.10 to 20.04 is unclear.
However, there's a comment over at discourse.ubuntu.com worth reading. Quoting from there:
This just sounds like a feature request in the upgrade tool.
Essentially if the user has already removed a snap, and installed the
deb of the application, then an upgrade happens they get a thing back
that they removed.
However, I’m not sure the upgrade tools have the intelligence to know
that you previously removed the snap, and don’t want it back. Further,
the point of the upgrade tool is to take you to the new release. Those
snaps are part of the new release. I have a feeling if you removed
firefox from 19.04 and upgraded to 19.10, you’d get firefox back.
Which is a similar use case that you’re describing.
That comment indicates that snaps should be reinstalled which isn't what we, who purged
(I would also point out that before upgrading to 19.10, I purged several fonts I don't need. They too were not re-installed.)
What makes these snap applications better?
Martin Wimpress is the new Desktop Director of Ubuntu. He talks about the rationale behind snaps in this video: Future of Ubuntu & Unpacking Snaps with Martin Wimpress | Part 2 | IG Talks ep. 4.