Configuring this options allows you to keep your database in sync with mailchimp if user will decide to resign from subscribing your mailing list through MailChimp link.
To create such a webhook on mailchimp side follow this official mailchimp article
Plugin ready endpoint is defined as follows:
bitbag_sylius_mailchimp_plugin_webhook:
path: /mailchimp/webhook
On webhook configuration on mailchimp use following Webhook URL:
https://yourdomain/mailchimp/webhook/?qsecret=<WEBHOOK_SECRET>
Used WEBHOOK_SECRET
on Mailchimp side should be the same as MAIL_CHIMP_WEBHOOK_SECRET
from .env file on Your project.