/usr/share/lightdm/lightdm.conf.d/50-disable-log-backup.conf is in lightdm 1.18.1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | [LightDM]
# Disable log backup - we use logrotate instead
backup-logs=false
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