When a contact unsubscribes from receiving emails from you on the email preferences page they are asked to provide a reason on the unsubscribe confirmation page. Answering this poll is optional.

Collection of data started on June 22, 2023 when this feature was made available.

Report for each email campaign, near the bottom of the email campaign report page

Report for each contact, on the contact details page

Aggregate report for the entire account, on the list report page