Mailchimp has released new plans. With the new pricing all subscribed, unsubscribed and non-subscribed contacts count towards the amount you pay each month (cleaned contacts do not count towards billing).
In regards to reducing your Mailchimp monthly spend, below I describe the difference between deleting and archiving contacts in Mailchimp.
What is the difference between deleting and archiving a contact?
Deleting a Mailchimp contact removes all data about that contact and removes evidence that the contact was in your audience (list).
Archiving a contact means that the contact no longer shows in your list (although you can view contacts through the Manage contacts menu item). Even although you won’t see the archived contact in the usual audience view, that contact is still in your list; you just can’t market to that person.
Whether you delete or archive a contact, you’ll no longer be billed for that contact.
When You Should Archive and not Delete Mailchimp Contacts
Deleting a contact may be needed for privacy or other legal reasons (e.g. GDPR). Other than for these reasons there is little reason to delete a contact.
The most obvious reason to archive rather than delete a contact is when new contacts are being added to your audience; if the contact is archived then the contact won’t be inadvertently added as a new contact. If you delete a contact then attempt to add that contact to your audience you will add the deleted contact to your list.
Adding a contact to a Mailchimp audience in error will result in that contact counting towards the amount you pay Mailchimp each month. It is also possible that you will contravene spam regulations as if you delete an unsubscribed contact then perform an import, the contact will be shown as subscribed in your audience.
In summary, unless required by law, it is probably safer to archive rather than delete contacts.