https://blog.herokuapp.com/evolution-of-heroku-cli-2008-2017 Their version 6, started with Ruby, then go/node, then pure node.

https://github.com/heroku/cli "This is the next generation Node-based Heroku CLI. The goals of this project were to make plugins more flexible, remove Ruby as a runtime dependency, and make the CLI faster. [...] It has identical functionality to the old Ruby CLI."