Importing Unsubscribed Users via CSV
Go to Voting board settings > Scroll down to Access > Open Voters’ list block.
Under Voters’ list, click Import list of Voters (CSV).
Prepare a CSV file with three columns Name, Email, and with the value 1 in the third column.
firstname.lastname@example.org,Olivia Johnson,1 email@example.com,John Smith,1
Exporting Unsubscribed Users to CSV
Under Voters’ list, click Export to CSV.
Click on the Run Export button, and wait for the file to be generated. Once generated, you can download the file.
Unsubscribed users will have a non-empty value in the all_unsubscribed_at field. This field contains the date the user unsubscribed.
Importing Unsubscribed Users via API
Find the article here – Open API.