This is what the Mandrill support page says:
But obviously this only applies if you have the Mandrill unsubscribe footer enabled.
I would suggest you don’t need it as you should already have the Discourse one which looks like this:
###What happens when someone unsubscribes?
When someone unsubscribes using a Mandrill unsubscribe link, the information is sent back to Mandrill to process . The message status is changed to Unsubscribe, and a sender-level rejection is added for that recipient address.
While the email is on the Rejection Blacklist, if you're using the same 'from' address on emails sent through Mandrill, any emails sent to that recipient will be rejected. The rejection is temporary, and in many cases it doesn't make sense for people to unsubscribe from transactional emails. However, this allows senders to get feedback about how people are engaged with their emails and the rejection prevents continued sending to someone who has signaled they don't wish to receive more emails.
You may remove an email address from the Rejection Blacklist list for a small reputation hit. Recipients shouldn't be removed in bulk from the rejection list since they're there because of a bounce, unsubscribe, or abuse complaint. Mistakes happen, or recipients don't always understand, so removing recipients from the rejection list is possible.
Note: The MailChimp
*|UNSUB|* merge tag would need to be modified to include a URL for the Mandrill unsubscribe merge tag if you're already using a MailChimp template.