X11 has been, and still is, a vital piece of technology at the core of professional Unix-like workstations since decades. It has a proven track record of supporting enterprise-grade applications with long-term protocol stability and platform compatibility. It has matured over decades. XLibre is an actively developed fork of the X.Org X11 server, initiated by the most active X.Org developer and supported by the open source community.
An incompatible alternative, Wayland, is being aggressively pushed by IBM = Red Hat = Gnome = Fedora = freedesktop.org. However, it is not ready to succeed X11 as it its governance model leads to never-ending discussions and prevents even the most essential functionality from existing. Think twice before abandoning Xorg. Wayland breaks everything!
It is time that the open source community reclaims what was ours to begin with. This page lists distributions supporting XLibre so that you can make an informed choice.
--> Table has moved to https://github.com/X11Libre/xserver/wiki/Are-We-XLibre-Yet%3F <--
Worth noting, SDDM is expected to be replaced within KDE with its own home-brew Login Manager soon (they reckon 6.5 but I don't see it happening before 6.6 at least), and just about all people who tend to use SDDM are those running KDE. With that in mind, it is uncertain whether there will be motivation to continue maintaining SDDM, or how it will be maintained. Probably may still be maintained as a desktop-agnostic display manager. That said, if it works with X.org then it should work with Xlibre anyway for the time being.
There is also slim/slimski for X11 environments. I am not sure if they support any Wayland sessions as I haven't tested that scenario myself.