When booting the systemd.volatile=overlay, the system drops to emergency mode while in initramfs phase, because by default the overlay kernel module is not embedded in the initramfs.

The NEWS file in systemd repository indicates that systemd.volatile=overlay exists, but there is no man page on it, for example systemd-fstab-generator and systemd-volatile-root.service discuss about having systemd.volatile=yes only.

Please confirm whether having systemd.volatile=overlay is supported or not.
If it is supported, please update the man pages accordingly and indicate that the initramfs must then be created with overlay module embedded.