/etc/xdg/xdg-xubuntu/ is in xubuntu-default-settings 16.04.6.
This file is owned by root:root, with mode 0o755.
..
/etc/xdg/xdg-xubuntu/Thunar/
/etc/xdg/xdg-xubuntu/Thunar/uca.xml
/etc/xdg/xdg-xubuntu/autostart/
/etc/xdg/xdg-xubuntu/autostart/xfce4-tips-autostart.desktop
/etc/xdg/xdg-xubuntu/compton.conf
/etc/xdg/xdg-xubuntu/menus/
/etc/xdg/xdg-xubuntu/menus/xfce-applications.menu
/etc/xdg/xdg-xubuntu/menus/xfce-settings-manager.menu
/etc/xdg/xdg-xubuntu/orage/
/etc/xdg/xdg-xubuntu/orage/oragerc
/etc/xdg/xdg-xubuntu/xfce4/
/etc/xdg/xdg-xubuntu/xfce4/helpers.rc
/etc/xdg/xdg-xubuntu/xfce4/panel/
/etc/xdg/xdg-xubuntu/xfce4/panel/default.xml
/etc/xdg/xdg-xubuntu/xfce4/terminal/
/etc/xdg/xdg-xubuntu/xfce4/terminal/terminalrc
/etc/xdg/xdg-xubuntu/xfce4/whiskermenu/
/etc/xdg/xdg-xubuntu/xfce4/whiskermenu/defaults.rc
/etc/xdg/xdg-xubuntu/xfce4/xfconf/
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/parole.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/thunar-volman.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/thunar.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfce4-desktop.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfce4-keyboard-shortcuts.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfce4-notifyd.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfce4-power-manager.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfce4-session.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfprint.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xfwm4.xml
/etc/xdg/xdg-xubuntu/xfce4/xfconf/xfce-perchannel-xml/xsettings.xml