building on the above I’ve made this template to use in a dismiss all script:
dismiss_all:
alias: 'Dismiss all Notifications'
icon: mdi:message-bulleted-off
sequence:
service: persistent_notification.dismiss
data_template:
notification_id: >
{%- set ns = namespace(notifications = '') %}
{%- for item in states.persistent_notification %}
{%- set id = item.object_id %} #previously used entity_id....
{%- set d = ', ' if ns.notifications | length > 0 else '' %}
{%- set ns.notifications = ns.notifications ~ d ~ id %}
{%- endfor %}
{%- if ns.notifications|length|int == 0 %} None
{% else %}
{{- ns.notifications }}
{%- endif %}
which shows the notifications as I would have thought they needed to be in a comma separated list, to use that in the notification-id field for the service:
however, clicking the script, (or using the developers-tools/service), nothing happens, not even a log entry to say what’s wrong 
@petro would you know how to handle/solve this?
update
found this python_script by @pnbruckner which works perfectly, thanks! (isn’t object_id available in python for the notification_id ? )
Would still like to understand why the above jinja template doesn’t work in the script setting.