All new Common Runtime custom domains get a <customdomain>.herokudns.com stable CNAME
Change effective on 12 December 2016
Newly created Common Runtime custom domains will automatically receive a <customdomain>.herokudns.com CNAME. This is intended to simplify the DNS burden on customers, reducing the number of times you need to repoint your domain at the Heroku stack.
You can use this CNAME for unencrypted traffic. For Hobby tier and above applications, you can also use this CNAME for SNI SSL termination as well.
Note: If your application also has the SSL Endpoint Add-on, the public API (info and list), Dashboard, and heroku domains from the console, will only show the herokudns.com cname.
To see all of the endpoints for terminating your app, use heroku certs from the console.