/etc/fonts/conf.d/10-powerline-symbols.conf is in fonts-powerline 2.6-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/fonts/conf.d/../conf.avail/10-powerline-symbols.conf
/etc/fonts/conf.d/../conf.avail/10-powerline-symbols.conf