I have tested this package for released xenial, bionic, focal and hirsute with the following script:

----------------------
#!/bin/sh
set -x

series=hirsute
name=$series-dev


function generate_motd_message() {
    message=$1
    echo "-----------------------------------------------------------"
    echo $message
    lxc exec $name -- /usr/lib/update-notifier/update-motd-updates-available --force
    lxc exec $name -- update-motd
    echo "-----------------------------------------------------------"
}

function turn_distro_into_esm_mode() {
    # guarantee that xenial distro is on ESM mode
    lxc exec $name -- sed -i 's/is_esm_supported and is_not_currently_supported/True #comment/' /usr/lib/update-notifier/apt-check
}

function turn_distro_into_non_esm_mode() {
    # guarantee that xenial distro is on ESM mode
    lxc exec $name -- sed -i 's/True #comment/False/' /usr/lib/update-notifier/apt-check
}

function setup_update_notifier() {
#    lxc exec $name -- add-apt-repository ppa:lamoura/update-notifier-test-ppa -y
#    lxc exec $name -- apt update
#    lxc exec $name -- apt install update-notifier-common -y
#    lxc exec $name -- apt update
#    lxc exec $name -- apt install python3-distro-info
#    lxc file push data/apt_check.py $name/usr/lib/update-notifier/apt-check
    lxc exec $name -- sh -c "cat <<EOF >/etc/apt/sources.list.d/ubuntu-$series-proposed.list
    deb http://archive.ubuntu.com/ubuntu/ $series-proposed restricted main multiverse universe"
    lxc exec $name -- apt update
    lxc exec $name -- sh -c "apt install update-notifier -yq > /dev/null"
}

function install_ua() {
    lxc exec $name -- add-apt-repository ppa:ua-client/daily -y
    lxc exec $name -- sudo apt-get update
    lxc exec $name -- sudo apt-get install ubuntu-advantage-tools -y
    lxc exec $name -- ua version
    lxc exec $name -- sudo apt-get update
}

function ua_attach() {
    lxc exec $name -- sed -i 's/contracts.can/contracts.staging.can/' /etc/ubuntu-advantage/uaclient.conf
    lxc exec $name -- ua attach $UACLIENT_BEHAVE_CONTRACT_TOKEN_STAGING
}

function ua_detach() {
    lxc exec $name -- ua detach --assume-yes
}

function generate_ua_motd_messages() {
    lxc exec $name -- python3 /usr/lib/ubuntu-advantage/ua_update_messaging.py
}

function install_update_motd() {
    lxc exec $name -- apt install update-motd -yq
}

function turn_esm_apps_into_non_beta() {
    lxc exec $name -- sh -c "echo 'features:\n  allow_beta: true' >> /etc/ubuntu-advantage/uaclient.conf"
}

function update_contract_effectiveto() {
    operation=$1
    num_days=$2
    replace_date=$(date -d "$date $operation$num_days days" +"%Y-%m-%dT00:00:00Z")
    echo $replace_date

    lxc exec $name -- sed -i "s/\"effectiveTo\": \"[^\"]*\"/\"effectiveTo\": \"$replace_date\"/g" /var/lib/ubuntu-advantage/private/machine-token.json
}


lxc delete --force $name
lxc launch ubuntu-daily:$series $name
sleep 10

setup_update_notifier
install_update_motd
generate_motd_message "$series: ua not installed"
install_ua
turn_esm_apps_into_non_beta
generate_ua_motd_messages
turn_distro_into_esm_mode
generate_motd_message "$series esm: ua not attached"
ua_attach
generate_ua_motd_messages
generate_motd_message "$series esm: ua attached"
turn_distro_into_non_esm_mode
ua_detach
generate_ua_motd_messages
generate_motd_message "$series non-esm: ua not attached"
ua_attach
generate_ua_motd_messages
generate_motd_message "$series non-esm: ua attached"
update_contract_effectiveto - 30
generate_ua_motd_messages
generate_motd_message "$series esm: ua attached - contract expired"
update_contract_effectiveto - 5
generate_ua_motd_messages
generate_motd_message "$series esm: ua attached - contract grace period"
update_contract_effectiveto + 5
generate_ua_motd_messages
generate_motd_message "$series esm: ua attached - contract expire soon"

set +x
-------------------------

I can confirm that on all release, the package now in proposed is working as expected