Plugin in my panel seems to be /usr/lib/x86_64-linux-gnu/xfce4/panel/plugins/libpulseaudio-plugin.so
Another name for this is the xfce4-pulseaudio-plugin, so that is right, actually - it is the same plugin I’ve mentioned. It prompts the option to use pavucontrol as the mixer.
If you check the General Tab in Properties, you can see the first option in Behaviour - “Enable keyboard shortcuts for volume control”. This isn’t working for me.
I wanted to log XFCE plugins, but I’m not really sure on how it should be done.