This article also answers:

Can customers view the cancellation reason provided by them?

--------------------------------------------------------------------------
The customer's feedback when they cancel through customer portal is added as a comment at the bottom of that particular subscription's details page. See screenshot below:



                                         
Notes:
  • These comments are for the merchant's reference and hence displayed within the web interface and not to the customer.
  • This answer is applicable to both V1 and V2 customer portals.