Got it
OK so {{GET_unsubscribe_newsletter.data}} is what you want to use against your own db.
Your query then should go against YOUR db Resource (Dev Database) and that is what is NOT working when using IN.
So try using ANY like so:
SELECT *
FROM
users as U
LEFT JOIN user_communication_preferences as UCP on U.id = UCP.user_id
where
email = ANY ({{GET_unsubscribe_newsletter.data}})