The "Unsubscribe" option/button is only available in email and in-app templates.
To return this option, follow these steps:
Go to the Campaigns page and click on an email or in-app campaign to access the campaign builder.
Next, in the Template section of your campaign, click the template that is assigned to the campaign.
Finally, in the Template builder, find the Layout Settings section in the left sidebar and tick the "Unsubscribe page" option.