Comment # 1 on bug 1008844 from Wolfgang Bauer

(In reply to Donald Curtis from comment #0)
> Restrict the KDE Plasma 5 user's options for Session Logout to "only logout"
> -- no system shutdown or restart.
> "Confirm Session Logout" is checked.
> 
> SDDM then offers all possible options -- "Logout"; "System Shutdown";
> "System Restart".

That's not SDDM, but KDE's (Plasma's) logout "greeter".

SDDM cannot respect the user's settings in any case, as it is running outside
the user session, and as a different user.

> Yes, yes, I know -- possibly an upstream issue

Yes, it is an upstream issue, and it would probably be good to report it
upstream. I wanted to do that already myself, but haven't found the time yet...