You’re reading the manual for Guix 1.4.0, which was released more than a few years ago! You might find the development version of the manual more helpful for documentation on plasma-desktop-service-type: https://guix.gnu.org/manual/devel/en/html_node/Desktop-Services.html#index-plasma_002ddesktop_002dservice_002dtype

I have Plasma configurations here, although it’s been a while since I’ve used them (am on GNOME at the moment): https://github.com/aurtzy/guix-config/blob/master/modules/my-guix/mods/desktop-environment.scm#L110

Also, if you downloaded the 1.4.0 installer, you might have to guix pull (so the current state of Guix’s plasma is made available) before trying to do any reconfiguring.

FWIW, the GNU Project makes an argument against LGPL if you believe your software “provides a significant unique capability”: https://www.gnu.org/licenses/why-not-lgpl.html