I installed a copy of Debian testing with Gnome 3.28 in a virtual machine.
Both contrib and non-free repositories are disabled.
Even with gnome-software-plugin-snap installed, Gnome Software in Debian
doesn't recommend any non-free software, as previously reported. So in a
correctly configured distribution, gnome-software-plugin-snap by itself does
neither good nor harm.
However, when I tried to search snap packages using "snap find" command, it
DOES return (suspected) non-free software such as spotify. I also tried to
run "snap find google" and got a long list of suspected non-free software
related to Google's SaaSS.
Though Debian users cannot install such non-free software from the Gnome
Software's graphical front end, it's still considered a potential freedom
issue, because users can be lured to try to use those non-free software.
Note: output for command "snap find spotify"
spotify
tizonia
vsslagent
magnesium
