We found that the clients cant unsubscribe mailchimp list by going to edit account details in whmcs client area . After clicking the no option they didn't get removed from mailchimp list, when enquired whmcs support about this they told this is because mailchimp didn't give them the api for this purpose and after contact with mailchimp they said " you can unsubscribe someone via our API. You can do so by using the endpoint: https://developer.mailchimp.com/documentation/mailchimp/reference/lists/members/#edit-put_lists_list_id_members_subscriber_hashYou
will need to change the status parameter to unsubscribed and from there you should be all set.
Once this is complete, WHMCS can update their integration" I told this to whmcs support . They adviced me to add as new topic here.