Bringing in the new upstream version of shim-signed also impacts livecd-rootfs, because the ubuntu-cpc scripts in livecd-rootfs have hard-coded logic around the UEFI filenames. So we need a fixed livecd-rootfs in each of the LTS releases before releasing the SRU of shim-signed+grub2 to those releases.
Bringing in the new upstream version of shim-signed also impacts livecd-rootfs, because the ubuntu-cpc scripts in livecd-rootfs have hard-coded logic around the UEFI filenames. So we need a fixed livecd-rootfs in each of the LTS releases before releasing the SRU of shim-signed+grub2 to those releases.