Summary
Chargebee allows you to capture the cancellation reason as feedback when your customers cancel their subscriptions from the customer portal. This feedback is stored as a comment under the respective subscription. You can access this feedback from the UI or via API.
To obtain the feedback to a canceled subscription,
- Open the Subscriptions details page, scroll down to the Comments and Attachments section, and view the feedback. To export these, refer to this article. 
- You can use Chargebee's API, using the Subscription Id called List Comment API (Filter parameter: entity_type= Subscription, entity_id= Id of Subscription). 
Note: There is no instant email notification setup that will include customer feedback.
Related articles and Documentation
Is it possible to create a subscription in the Canceled state?
