Neither GNOME nor Plasma depend on NetworkManager, do they?
Not directly, but distros may choose to create a dependency.
On Debian, installing recommended dependencies is enabled by default and disabling them can lead to all sorts of errors and missing functionality.
gnome-shell recommends gnome-control-center, which recommends network-manager-gnome, which depends on network-manager.
So unless you go out of your way to install a very minimal system, it gets pulled in.