Note: there were some changes regarding display managers recently. The changes involved updating and cleaning up code in ‘/usr/lib/X11/display-manager’, ‘/etc/alternatives/’ , and other places. This work also facilitated the decoupling of Wayland-only projects from dependencies on ‘xdm’, the Xorg server, and related components, eliminating the need for their installation.
https://en.opensuse.org/openSUSE:DisplayManagerRework
This allows the various display managers to be managed separately via their own respective systemd unit files (like other distros do), and alleviates the need for ‘update-alternatives’ with respect to selecting the preferred DM.
The ‘update-alternatives’ framework will still work for those using 'display-manager-legacy.service though…
systemctl status display-manager-legacy