seife@strolchi:~> rpm -qf /usr/lib/systemd/system/display-manager.service
xdm-1.1.14-4.3.x86_64
seife@strolchi:~> grep OnFai /usr/lib/systemd/system/display-manager.service
OnFailure=plymouth-quit.service

I killed the lightdm login screen with ctrl-alt-backspace and it did not come
back again.

Investigating, I found this in the journal:

Sep 30 18:34:57 strolchi lightdm[1475]: pam_unix(lightdm-greeter:session):
session closed for user lightdm
Sep 30 18:34:57 strolchi systemd[1]: session-1.scope: Deactivated successfully.
Sep 30 18:34:57 strolchi systemd[1]: session-1.scope: Consumed 1.622s CPU time.
Sep 30 18:34:57 strolchi systemd-logind[1135]: Session 1 logged out. Waiting
for processes to exit.
Sep 30 18:34:57 strolchi systemd-logind[1135]: Removed session 1.
Sep 30 18:34:57 strolchi display-manager[1353]: Starting service lightdm
Sep 30 18:34:57 strolchi systemd[1]: display-manager.service: Main process
exited, code=exited, status=1/FAILURE
Sep 30 18:34:57 strolchi systemd[1]: display-manager.service: Failed with
result 'exit-code'.
Sep 30 18:34:58 strolchi systemd[1]: display-manager.service: Triggering
OnFailure= dependencies.
Sep 30 18:34:58 strolchi systemd[1]: display-manager.service: Failed to enqueue
OnFailure= job, ignoring: Unit plymouth-quit.service not found.
Sep 30 18:34:58 strolchi systemd[1]: display-manager.service: Consumed 1.290s
CPU time.

I think however that the OnFailure failure is not the reason for the problem
but that the display-manager.service should have a "Restart=on-failure" line.

current tumbleweed, xfce4 desktop, lightdm login manager
strolchi:~ # head -3 /etc/os-release
NAME="openSUSE Tumbleweed"
# VERSION="20230927"
ID="opensuse-tumbleweed"