Short answer
In email subscription settings, Unsubscribe feedback survey asks contacts why they leave. HubSpot's KB says it appears only when contacts unsubscribe from all marketing emails, not from one subscription type. Reasons show in the email's post-send summary.
1. The setting
It is on in my demo portal, next to Don't send to unengaged contacts.

2. Use the answers
- Too frequent: review frequency caps.
- Not relevant: tighten segments.