Short answer
HubSpot's KB: you can add an unsubscribe link to one-to-one emails, from records or sequences. Clicking it opens the unsubscribe page for that specific subscription type, with a link to manage other preferences. If data privacy settings are off, these links aren't included by default. Super Admin and Account access permissions are needed to manage the defaults.
1. Subscription settings
Email subscription settings in my demo portal.

2. Subscription types
See subscription types.