/usr/share/lxc/config/archlinux.userns.conf is in lxc-templates 2.0.7-0ubuntu1~16.04.2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # This derives from the global userns config
lxc.include = /usr/share/lxc/config/userns.conf
|