It remains very popular amongst Ruby/Rails users, presumably due to less switching overhead between Ruby and CoffeeScript vs JavaScript.
ES6 has provided a lot of the features that people were hoping for in JS, and CoffeeScript doesn't seem to be worth the effort transpiling from anymore. It used to be the game in town for "better" JS, but TypeScript seems to have taken that position firmly.
CoffeScript is endorsed by DHH and so it's more of a first class citizen than JavaScript in Rails. However it's not something you have to use. I never really liked using it and I easily managed to keep using JavaScript in all these years since Rails 3.1. I hope it's going to the deprecation bin soon but I believe that Basecamp is using it a lot. DHH won't have added it to Rails if that was not the case.
ES6 has provided a lot of the features that people were hoping for in JS, and CoffeeScript doesn't seem to be worth the effort transpiling from anymore. It used to be the game in town for "better" JS, but TypeScript seems to have taken that position firmly.