This is wrong. For legacy code there is Xwayland and the blog post actually describes it.

I'm using Wayland on Archlinux and despite I run currently no legacy software Xwayland is active:

    $ xlsclients 
    gsd-xsettings
    mutter-x11-frames

Therefore Xwayland is running and consuming 164 Megabyte of RSS. Despite I don't run applications depending on X11. I suspect the file /etc/xdg/autostart/org.gnome.SettingsDaemon.XSettings.desktop is the cause:

    [Desktop Entry]
    Type=Application
    Name=GNOME XSettings
    Exec=/usr/lib/gsd-xsettings
    OnlyShowIn=GNOME;
    NoDisplay=true
    X-GNOME-Autostart-Phase=Initialization
    X-GNOME-Autostart-Notify=true
    X-GNOME-AutoRestart=true
    X-GNOME-HiddenUnderSystemd=true

If I guess it right this should help X11 based applications to get the needed settings. Probably /org/gnome/mutter/experimental-features with autoclose-xwayland is what I want. I assumed that is already the default - wrong assumption. Xwayland remains available for stuff like legacy software (various games from Steam) and doesn't consume resources when not required. I wouldn’t be surprised if legacy games ship in future (ten years or more…) with a built-in Xwayland-Wrapper or something similar.