Comment 9 for bug 1724944

I am closing this bug. I believe the problem was that you didn't uncomment the [org/gnome/login-screen] line.

To disable the user list on the login screen in Ubuntu 18.04 and newer:
1. Edit /etc/gdm3/greeter.dconf-defaults

You can use your preferred editor, but I suggest:

gedit admin:///etc/gdm3/greeter.dconf-defaults

2. Remove the # from the front of these two lines to uncomment them:
[org/gnome/login-screen]
 disable-user-list=true

3. Save the file

4. Reboot your computer.
On Ubuntu 18.10 and newer, you can probably skip rebooting and just run this command:

sudo systemctl reload gdm