#TIL Never delete a contact on Mailchimp – not even when you’re just testing a sign-up form, unless (see below). As per Mailchimp support:
Deleting a contact is a permanent action that is intended to prevent the address from being re-added in the future.
You will not be able to re-add the same email address to the audience manually, import it, nor re-subscribe through a third-party signup form. The only way for a deleted contact to be re-added is if they sign up through a Mailchimp-created signup form.