They are actually automatically archiving unsubscribed emails to keep free users under the limit.
> You can remain a free user so long as you have 2,000 or fewer contacts in your audience, and you’ll now have the new free plan features. If the new way of counting contacts causes your audience to exceed 2,000 contacts before June 15, 2019, we’ll automatically archive your unsubscribed and transactional contacts. After June 15, we’ll begin calculating your audience as described in Section 7B of our TOU, but you can always manually archive contacts to keep your audience under 2,000 contacts.
> You can remain a free user so long as you have 2,000 or fewer contacts in your audience, and you’ll now have the new free plan features. If the new way of counting contacts causes your audience to exceed 2,000 contacts before June 15, 2019, we’ll automatically archive your unsubscribed and transactional contacts. After June 15, we’ll begin calculating your audience as described in Section 7B of our TOU, but you can always manually archive contacts to keep your audience under 2,000 contacts.