How does the Sympa integration deal with unsubscribes
CiviCRM-Sympa Syncronization includes unsubscription pages (it was introduced with this commit). On « sympasync » configuration page (https://example.org/civicrm/a/#/sympa/admin), you’ll find the URL that can be added to your list’s footer. URL will look like https://example.org/civicrm/sympa/unsubscribe?gid=1.

Problem is that you can’t override the default unsubscription mechanism in Sympa, so you’ll need to disable the ability for users to unsubscribe in your list configuration (to force them to use sympasync’s link). If you let Sympa’s default mechanism, people will be able to unsubscribe (and it will be effective), but information won’t be pushed back to CiviCRM.

So by adding the unsubscription link at the end of each mail sent on your Sympa mailing list and by disabling the ability to unsubscribe in your list configuration, you’ll get what you want.