Comment 2 for bug 274822

Nope, it's not done, and won't be done in Ubuntu directly (rather in Debian).

I discussed with the Debian maintainer about that some time ago (when starting to package xfce4-panel 4.7, and thinking about the transition), and the split may create lots of issues with plugins: if we don't split, we can put (>=4.6.0) and (<< 4.7.0) in xfce4-panel.shlibs, to avoid plugins built against xfce4-panel 4.6 to be used with xfce4-panel 4.7.
If we split the package, a plugin built against xfce4-panel 4.6 would depend on libxfcepanel1, and a plugin built against xfce4-panel 4.7 on libxfce4panel1.0-3. But this would not prevent a plugin built against 4.6 from breaking badly if xfce4-panel 4.7 is installed (unless we add a Breaks: libxfcepanel1 in libxfce4panel1.0-3, but it looks a bit ugly).