I've finally finished the long-abandoned project that I've been meaning to build for a while.
If you use Stripe Billing for subscriptions, your customers can specify reasons why they cancelled (e.g. too expensive, not using it, switched to competitor, etc.). However, to access those, you either have to use Stripe Sigma or pull them from the API. I wanted to build a more convenient way to access those (and also act upon them).
I've submitted the app to Stripe's App Marketplace, but I have a limited number of test invites to send out if you're interested (I will happily waive your subscription for 3 months).