To prevent powering down the machine by accident, I have put following contents to /etc/sddm.conf and to /etc/sddm.conf.d/01-my-config:

   [General]
   HaltCommand=/bin/false
   RebootCommand=/bin/false

Those files are set to mode 0644 and the directories are set to mode 0755, so there should be no permission problems.

But "sddm --example-config" still gives:

   [General]
   # Halt command
   HaltCommand=/usr/bin/systemctl poweroff
   
   # Input method module
   InputMethod=qtvirtualkeyboard
   
   # Comma-separated list of Linux namespaces for user session to enter
   Namespaces=
   
   # Initial NumLock state. Can be on, off or none.
   # If property is set to none, numlock won't be changed
   # NOTE: Currently ignored if autologin is enabled.
   Numlock=none
   
   # Reboot command
   RebootCommand=/usr/bin/systemctl reboot

This is on opensuse Leap.15.1, which contains sddm-0.18.0-lp151.3.3.1.x86_64