List the contacts that unsubscribed through a campaign, with the reason they gave. This is the v3 equivalent of the v1 campaign_report_unsubscription_list action.

Only unsubscribes attributed to a message of the campaign are reported; unsubscribes recorded without a message (for example from an automation) are not. The response carries no meta.total.

Requires the Campaigns feature and access to the campaign; a campaign the user cannot see is reported as 404 so the endpoint does not disclose which campaign ids exist.