Hello. kde-gtk-config is already being patched to have optional gtk+:3 dep, which is nice. The next logical step would be to avoid pulling gtk+:2 unconditionally as well. Judging by kde-gtk-config-5.4.2-gtk3-optional.patch it requires a minimal amount of effort, but could be helpful for people who don't have gtk+:2 apps.
Patches welcome. It was easy to make gtk3 optional, not both - that requires bigger cmake changes since gtk3proxies uses content from gtkproxies.
Feel free to re-open once you spot a fix upstream, we'll certainly make it optional as soon as the option is provided by the build system.