networkmanager-qt bundles (in Targets.cmake files) a copy of networkmanager build-time dependencies (taken from pkg-config). At the same time networkmanager-qt public include files refer to libnm includes. And when libnm build flags change, networkmanager-qt has to be rebuilt. For example, libnm 1.10.2 added glib-2.0 include dirs, and all networkmanager-qt clients were failing to compile.
build-time dependencies -> build-time requirements
networkmanager doesn't define any subslots (just default implicit SLOT="0/0", indicating it won't change). Adding subslot operator won't help here in any way. That's not what they are for.
I'm not sure how likely it is a change like this will happen in the future. All we can do then will be adding appropriate min/max version definitions to networkmanager-qt. networkmanager ebuild does mention it would add subslot if libnm-{glib,util} were bumped, so there's that. If networkmanager USE config does change these pkgconfig derived deps that would be fixable by mirroring the respective USE in lockstep inside networkmanager-qt ebuild.