I using KDE Desktop and got SDDM display manager by default. But I before use MATE Desktop and by default comming with LightDM.
LightDM have support for VNC and I need that feature, here is procedure how to replace SDDM with LightDM (open console as root) :
apt install lightdm apt install slick-greeter
Now reboot PC and can complete remove sddm package:
apt purge sddm