From a glance this looks like a formalization of what already existed at: https://api-docs.heroku.com/ and which obviously the command-line client uses (although the docs and command line/production API have drifted apart)

At Pogoapp we basically reverse-engineered our API from Heroku's command line client, but it's excellent to have Heroku make the API public and formal, because it should make that process much easier and set up Heroku's API as a good target for other PaaS hosts to standardize on (in the same way Ceph & RiakCS have S3-compatible REST APIs)