form to unsubscribe from a newsletter but from all
You should be able to do this by directing folk to eg  www.yourdomain.org/civicrm/mailing/subscribe and if you have your Mailing List groups set up correctly (ie as public) then they should be able to unsub.

for folk using Drupal you can also do this via Webform

also review instructions at https://docs.civicrm.org/user/en/latest/email/set-up/