Hi everyone,
just to clarify: libxfce4ui is not really used that much by panel plugins
(apart from their properties dialogs), but by applications. One of the
bigger remaining applications to be ported before 4.14 was Ristretto, but
it has since seen a Gtk3 release (v0.10.0). That was one of the reasons to
leave Gtk2 support in the codebase and also enabled by default.
We also have a list upstream which we tried to keep up-to-date during the
4.14 cycle, where you can see that some plugins or apps are still unported.
The notes, time-out and mail-watch plugins as well as xfburn (although
there is some WIP stuff).
https://wiki.xfce.org/releng/4.14/roadmap (toward the bottom of the page)
During the 4.16 cycle we will drop Gtk2 support upstream.