Hide user list on login screen..

If your system uses AccountsService, you can not hide a user from the greeter screen by reconfiguring lightdm because it defers to AccountsService. That is stated very clearly in the comments in /etc/lightdm/users.conf.

What you need to do instead is to reconfigure AccountsService.

To hide a user named XXX, create a file named

/var/lib/AccountsService/users/XXX

containing two lines:

[User] SystemAccount=true

If the file already exists, make sure you append the SystemAccount=true line to the [User] section.

The change takes effect after reloading AccountsService:

sudo systemctl restart accounts-daemon.service