/usr/share/lightdm/lightdm.conf.d/50-xubuntu-numlock.conf is in xubuntu-default-settings 18.04.6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [Seat:*]
greeter-setup-script=xubuntu-numlockx
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