This ArchLinux package provides an initcpio hook to copy your /etc/shadow in your initramfs, allowing you to debug initramfs in emergency mode instead of getting

Cannot open access to console, the root account is locked.
See sulogin(8) man page for more details.

1. Install the package:

yay -S initcpio-hook-shadowcopy

(any AUR helper will do, or you can build it manually with makepkg)

2. Add the hook to your /etc/mkinitcpio.conf:

HOOKS=(base systemd shadowcopy ...)

Note: the systemd hook overwrites /etc/shadow, so shadowcopy must be placed after it.

3. Rebuild your initramfs:

# mkinitcpio -P

You can then reboot, and login to your root account in emergency mode.

Copying your /etc/shadow to initramfs when you use an encrypted root partition has the downside of leaking every account passwords outside it. If you use an encrypted root partition, you can use a different password for the root account in initramfs by creating a file /etc/shadow.initramfs using the same format as /etc/shadow.

You can create the file like this:

echo "root:$(mkpasswd 'YOUR_PASSWORD'):::::::" > /etc/shadow.initramfs

If you want to generate a password that can be typed on azert and qwerty keyboards alike, you can use this script.