Xfce4 was originally in GTK2. But after a recent upgrade to version 4.12, I find it is halfway GTK2 and halfway GTK3. Which applications are currently ported to GTK3 and which are currently not yet?
There is no such list - and even if one existed, it would be outdated fairly quickly.
This is the case even if you're only counting "official" xfce apps, ignoring the thousands more GTK2 & GTK3 programs that can be run on a system running xfce.
The important thing to know is that, regardless of whether you use xfce or not, if you run GTK apps you will need a theme compatible with both GTK2 and GTK3 installed. This will be the case for years to come - don't expect this to change any time soon.