Comment # 6 on bug 1208268 from Felix Miata

# echo $PATH
/sbin:/usr/sbin:/usr/local/sbin:/root/bin:/usr/local/bin:/usr/bin:/bin
# which startice
/usr/local/bin/startice
# cat /usr/local/bin/startice
#!/bin/sh
WINDOWMANAGER=/usr/bin/icewm startx
# which k5start
/usr/local/bin/k5start
# cat /usr/local/bin/k5start
#!/bin/sh
WINDOWMANAGER=startplasma-x11 startx
#

Booted to graphical with IceWM as default xsession and logging in on XDM, in
the Xterm that opens, running startice or /usr/local/startice replaces the gray
screens and Xterm with a fresh IceWM session with toolbar and black desktop
background. Exiting that session returns the gray backgrounds and Xterm.
Running k5start instead, klauncher reports segfault.

Booting after causing:
# ls -Gg /etc/X11/xinit/xinitrc.common
-rw-r--r-- 1 5186 Feb 15 14:23 /etc/X11/xinit/xinitrc.common
# grep }.desktop /etc/X11/xinit/xinitrc.common
                for file in
${dir}/{default,plasma*,kde*,gnome*,xfce*,lxde*,icewm*,*}.desktop
#
didn't change behavior. XDM still starts X as described in comment #0:
# ps -A
...
  786 ?        00:00:00 xdm
  788 tty7     00:00:00 X
  846 ?        00:00:00 xdm
  861 ?        00:00:00 xconsole
  870 ?        00:00:00 systemd
  871 ?        00:00:00 (sd-pam)
  877 ?        00:00:00 xterm
...
#

After trying to reproduce this on another host, I did some more digging around
and found the following:
# journalctl | grep deprecated
Oct 26 18:17:21 ara88 udevadm[532]: systemd-udev-settle.service is deprecated.
Please fix display-manager.service not to pull it in.
Oct 27 11:37:47 ara88 udevadm[533]: systemd-udev-settle.service is deprecated.
Please fix display-manager.service not to pull it in.
Oct 27 11:43:18 ara88 udevadm[530]: systemd-udev-settle.service is deprecated.
Please fix display-manager.service not to pull it in.
Oct 27 11:46:27 ara88 udevadm[530]: systemd-udev-settle.service is deprecated.
Please fix display-manager.service not to pull it in.
Feb 14 19:24:39 ara88 [RPM][6901]: erase
dracut-mkinitrd-deprecated-055+suse.294.gc5bc4bb5-150400.3.8.1.x86_64: success
Feb 14 19:24:39 ara88 [RPM][6901]: install
dracut-mkinitrd-deprecated-055+suse.345.g8b8708cb-150500.1.2.x86_64: success
Feb 14 19:24:39 ara88 [RPM][6901]: erase
dracut-mkinitrd-deprecated-055+suse.294.gc5bc4bb5-150400.3.8.1.x86_64: success
Feb 14 19:24:39 ara88 [RPM][6901]: install
dracut-mkinitrd-deprecated-055+suse.345.g8b8708cb-150500.1.2.x86_64: success
Feb 14 20:58:57 ara88 udevadm[524]: systemd-udev-settle.service is deprecated.
Please fix display-manager.service not to pull it in.
# systemctl restart xdm
Failed to restart xdm.service: Unit systemd-udev-settle.service is masked.
#

Above reminded me I did what the journal wanted - wrongly. I disabled
systemd-udev-settle.service and masked it, then forgot about it.

# rpm -qf /usr/lib/systemd/system/systemd-udev-settle.service
udev-249.14-150400.8.19.1.x86_64
# systemctl cat xdm.service
# /usr/lib/systemd/system/display-manager.service
[Unit]
Description=X Display Manager
Requires=var-run.mount
Conflicts=getty@tty7.service plymouth-quit.service
After=ypbind.service gpm.service time-sync.target winbind.service acpid.service
Wants=remote-fs.target dbus.socket systemd-user-sessions.service
systemd-logind.service
After=remote-fs.target dbus.socket systemd-user-sessions.service
systemd-logind.service getty@tty7.service plymouth-quit.service
OnFailure=plymouth-quit.service

[Service]
Type=forking
PIDFile=/run/displaymanager.pid
ExecStart=/usr/lib/X11/display-manager start
ExecStop=/usr/lib/X11/display-manager stop
ExecReload=/usr/lib/X11/display-manager reload
KillMode=process

[Install]
WantedBy=graphical.target
Alias=xdm.service

# /etc/systemd/system/display-manager.service.d/override.conf
[Unit]
After=systemd-udev-settle.service
Requires=systemd-udev-settle.service
#

Override.conf created 2021-08-21:
https://bugzilla.opensuse.org/show_bug.cgi?id=1188954#c17

So I tried getting rid of it and rebooting:
# systemctl cat xdm.service
# /usr/lib/systemd/system/display-manager.service
[Unit]
Description=X Display Manager
Requires=var-run.mount
Conflicts=getty@tty7.service plymouth-quit.service
After=ypbind.service gpm.service time-sync.target winbind.service acpid.service
Wants=remote-fs.target dbus.socket systemd-user-sessions.service
systemd-logind.service
After=remote-fs.target dbus.socket systemd-user-sessions.service
systemd-logind.service getty@tty7.service plymouth-quit.service
OnFailure=plymouth-quit.service

[Service]
Type=forking
PIDFile=/run/displaymanager.pid
ExecStart=/usr/lib/X11/display-manager start
ExecStop=/usr/lib/X11/display-manager stop
ExecReload=/usr/lib/X11/display-manager reload
KillMode=process

[Install]
WantedBy=graphical.target
Alias=xdm.service
# systemctl cat display-manager.service
# /usr/lib/systemd/system/display-manager.service
[Unit]
Description=X Display Manager
Requires=var-run.mount
Conflicts=getty@tty7.service plymouth-quit.service
After=ypbind.service gpm.service time-sync.target winbind.service acpid.service
Wants=remote-fs.target dbus.socket systemd-user-sessions.service
systemd-logind.service
After=remote-fs.target dbus.socket systemd-user-sessions.service
systemd-logind.service getty@tty7.service plymouth-quit.service
OnFailure=plymouth-quit.service

[Service]
Type=forking
PIDFile=/run/displaymanager.pid
ExecStart=/usr/lib/X11/display-manager start
ExecStop=/usr/lib/X11/display-manager stop
ExecReload=/usr/lib/X11/display-manager reload
KillMode=process

[Install]
WantedBy=graphical.target
Alias=xdm.service
#

Login from XDM still is starting as described in comment #0, even with:
# head -n6 /etc/alternatives/default-xsession.desktop
[Desktop Entry]
Type=XSession
Exec=/usr/bin/startplasma-x11
TryExec=/usr/bin/startplasma-x11
DesktopNames=KDE
Name=Plasma (X11)
:(

Unless you have suggestions how to find what needs fixing, I'm ready to trash
this installation, install 15.5b anew, and close this.