Unblocking only affects future emails. You do not need to add the sender to Contacts for delivery, but it helps prevent misclassification.
If the emails are long gone, you must visit the "Vault of Filters" to find them: how to unblock emails on gmail
For a "mass pardon," you can check multiple boxes and click at the bottom. On the Gmail Mobile App Unblocking only affects future emails
If you have deleted all emails from the sender, you can still unblock them through your account settings. This method also allows you to unblock multiple people at once. Block an email address in Gmail - Computer - Google Help how to unblock emails on gmail