It works (see screenshot). Spotify and other proprietary software are also
recommended in Gnome Software Center (see other screenshot). I reported the
latter as a licensing issue
months ago, but had not actually attempted installing any proprietary to see
if it would work. Apparently it does.
Even after running
$ snap remove spotify
Gnome Software still says it's installed. When I try to remove it within
Gnome Software I get
"Detailed errors from the package manager follow: snapd returned status code
400: Bad Request"
Gnome Software is pretty buggy to begin with, so hopefully Spotify is indeed
no longer installed. I may reinstall Trisquel to be safe though.
Perhaps snapd should be temporarily removed until this is issue fixed.