The engines are there in the gtk2-common-themes snap and it plugs the gtk-2-engines content slot.
From what I was able to confirm is that the snap should be using gtk-common-themes to get the themes instead of gtk2-common-themes which carries only the engines now. I did disconnected the themes content plug and connected it to gtk-common-themes:gtk-2-themes, but I’m still seeing the same problem.
Further investigation shows that the snap sets GTK_PATH to $SNAP/lib/gtk-2.0 where the engines are mounted, but once the process is running:
$ sudo strings /proc/162354/environ|grep GTK_PAT
GTK_PATH=/snap/inkscape/7378/usr/lib/x86_64-linux-gnu/gtk-2.0/
It’s set differently. There is no pixmap engine in the Inkscape snap at that location, hence the warning. If that engine is required to render the theme, then GTK 2 will be unable to do so and will fall back to the default Win98 like theme.