I have seen that this is currently not a direct functionality and has already been raised for a feature request suggesting for its implementation.
Is there a workaround, that would not require any plugins/add-ons, within Automation that would somehow mimic the scenario of notifying non-licensed customers when their tickets are assigned?
I've tried enabling an automation that when "Ticket is created", then "Assignee" field is not empty, it would send an e-mail to the reporter and customers involved — it doesn't work.