nm-applet is built with --with-libnm-gtk. According to upstream (https://gitlab.gnome.org/GNOME/network-manager-applet/blob/master/NEWS), this library is deprecated since 1.8.20 and shouldn't be used. It also pulls in dbus-glib, which is also deprecated. Does libnm-gtk have any users in gentoo? In that case maybe hide it behind a use flag.
libnm-gtk is now gone with gnome-extra/nm-applet-1.8.24-r1. There is an explicit dbus-glib dep there for nm-applet as well, that seems entirely unnecessary, but didn't notice that. It'll go away naturally as the ebuild is ported to meson, or I'll try to remember if autotools is used for now still in the bump.