Unsubscribed contacts can be reinstated by a manager or owner on the team. Once they have been reinstated, they will be put in a paused indefinitely state to prevent sending emails to them immediately. You can set the contacts to resume email sending when you are ready to send them emails again.
You can reinstate unsubscribed contacts in your Data settings.
If you hover over the contact's status in a sequence, you can see that they have been paused after being resubscribed.