The Heroku Changelog is a feed of all public-facing changes to the Heroku runtime platform. While we announce all major new features via the Heroku blog, we're making small improvements all the time. When any of those improvements have any user-visible impact, you'll find them in the changelog.
Some recent examples of posts to the changelog include new versions of the Heroku CLI, a new error code, and changes to logging.
To get the latest on changes like these, visit the Heroku Changelog, or subscribe via feed or Twitter.