Longtime Fedora Silverblue user here, who recently jumped over to Kinoite (Atomic KDE). I typically enable autologin on my display managers because I use whole disk encryption and already need to enter my passphrase to decrypt and start the OS.

I discovered pretty quickly that SDDM's autologin feature isn't working under Fedora 40. LightDM also failed to start under Wayland on F40, regardless of which greeter I tried.

Long story short, I opted to use GDM since I knew its automatic login feature worked fine under Wayland. It's worth noting that KDE has it's own lockscreen mechanism, so you won't even see GDM unless you manually logout of your session. To try this yourself:

1. Install GDM: rpm-ostree install --apply-live gdm

2. Disable SDDM: sudo systemctl disable sddm

3. Enable GDM: sudo systemctl enable gdm

4. Reboot and select the Plasma session before logging in; this is required only once in order to establish to the default, otherwise GDM will load a broken GNOME session when autologin is enabled

5. Edit /etc/gdm/custom.conf and add the following under [daemon] (replacing username with your own):

    AutomaticLoginEnable = true
    AutomaticLogin = username
   

Voila! You will no longer need to enter your user credentials before loading the desktop.