From the contact card, you can view when a contact has been subscribed to a particular follow-up campaign and when they have been removed.

1. From the side navigation click on CONTACTS.
2. Search for and then click on a contact.
3. Click the FOLLOW-UP tab from the bottom right.
4. From the VIEW option, select to filter by ACTIVE, CANCELLED, or ALL campaigns.

This will show you a list of the campaigns and the date(s) when the contact was added or removed for each.