In what follows, the ubuntu-${series}-core-cloudimg-amd64-root.tar.gz tarball is the current tarball from https://partner-images.canonical.com/core/ and livecd.ubuntu-base.rootfs-${series}.tar.gz is a test build I made with -proposed enabled (see build pages at https://launchpad.net/~mwhudson/+livefs/ubuntu/disco/test/+build/165334 https://launchpad.net/~mwhudson/+livefs/ubuntu/cosmic/test/+build/165335 https://launchpad.net/~mwhudson/+livefs/ubuntu/bionic/test/+build/165336):

mwhudson@ringil:~/Downloads$ diff -u <(cat ubuntu-bionic-core-cloudimg-amd64-root.tar.gz | tar -tz  | sort) <(cat livecd.ubuntu-base.rootfs-bionic.tar.gz |  tar -tz | sort)
--- /dev/fd/63	2019-05-10 11:37:29.145681446 +1200
+++ /dev/fd/62	2019-05-10 11:37:29.149681443 +1200
@@ -86,19 +86,6 @@
 bin/znew
 boot/
 dev/
-dev/fd
-dev/full
-dev/null
-dev/ptmx
-dev/pts/
-dev/random
-dev/shm/
-dev/stderr
-dev/stdin
-dev/stdout
-dev/tty
-dev/urandom
-dev/zero
 etc/
 etc/adduser.conf
 etc/alternatives/
@@ -2812,24 +2799,6 @@
 var/lib/apt/
 var/lib/apt/extended_states
 var/lib/apt/lists/
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic_InRelease
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic_main_binary-amd64_Packages
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic_main_i18n_Translation-en
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic_restricted_binary-amd64_Packages
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic_restricted_i18n_Translation-en
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic-updates_InRelease
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic-updates_main_binary-amd64_Packages
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic-updates_main_i18n_Translation-en
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic-updates_restricted_binary-amd64_Packages
-var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_bionic-updates_restricted_i18n_Translation-en
-var/lib/apt/lists/auxfiles/
-var/lib/apt/lists/lock
-var/lib/apt/lists/partial/
-var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_bionic-security_InRelease
-var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_bionic-security_main_binary-amd64_Packages
-var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_bionic-security_main_i18n_Translation-en
-var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_bionic-security_restricted_binary-amd64_Packages
-var/lib/apt/lists/security.ubuntu.com_ubuntu_dists_bionic-security_restricted_i18n_Translation-en
 var/lib/apt/mirrors/
 var/lib/apt/mirrors/partial/
 var/lib/apt/periodic/
mwhudson@ringil:~/Downloads$ diff -u <(cat ubuntu-cosmic-core-cloudimg-amd64-root.tar.gz | tar -tz  | sort) <(cat livecd.ubuntu-base.rootfs-cosmic.tar.gz |  tar -tz | sort)
--- /dev/fd/63	2019-05-10 11:37:37.853671457 +1200
+++ /dev/fd/62	2019-05-10 11:37:37.853671457 +1200
@@ -86,7 +86,6 @@
 bin/znew
 boot/
 dev/
-dev/null
 etc/
 etc/adduser.conf
 etc/alternatives/
@@ -2788,9 +2787,6 @@
 var/lib/apt/
 var/lib/apt/extended_states
 var/lib/apt/lists/
-var/lib/apt/lists/auxfiles/
-var/lib/apt/lists/lock
-var/lib/apt/lists/partial/
 var/lib/apt/mirrors/
 var/lib/apt/mirrors/partial/
 var/lib/apt/periodic/
mwhudson@ringil:~/Downloads$ diff -u <(cat ubuntu-disco-core-cloudimg-amd64-root.tar.gz | tar -tz  | sort) <(cat livecd.ubuntu-base.rootfs-disco.tar.gz |  tar -tz | sort)
--- /dev/fd/63	2019-05-10 11:37:48.605659084 +1200
+++ /dev/fd/62	2019-05-10 11:37:48.605659084 +1200
@@ -1,7 +1,6 @@
 bin
 boot/
 dev/
-dev/null
 etc/
 etc/adduser.conf
 etc/alternatives/
@@ -2815,9 +2814,6 @@
 var/lib/apt/
 var/lib/apt/extended_states
 var/lib/apt/lists/
-var/lib/apt/lists/auxfiles/
-var/lib/apt/lists/lock
-var/lib/apt/lists/partial/
 var/lib/apt/mirrors/
 var/lib/apt/mirrors/partial/
 var/lib/apt/periodic/

This is exactly what we wanted to see, so marking verification-done for all series.