Upstream (the actual author) is https://docs.xfce.org/panel-plugins/xfce4-embed-plugin 
according to our page https://guix.gnu.org/packages/xfce4-embed-plugin-1.6.0/ .

Their latest release was in 2015, so I don't think they adapted to the new libxfce4ui.

That's too bad.

There are multiple possible ways to proceed:

(1) Delete xfce4-embed-plugin, if possible (if nobody is using it and if it has
security problems anyway etc), or
(2) Keep both packages of libxfce4ui.  Use the old package for
xfce4-embed-plugin and the new package for the other dependents.

Please make sure to use ./pre-inst-env guix lint xfce4-embed-plugin libxfce4ui
(with the old unchanged version) to find out whether there are known security
problems with it.

If not, I'd prefer (2).