Ruby on Rails is the stack I've felt most comfortable in for well over a decade and it's always great to see it's continued progress!
That said, I do think it's unfortunate that Rails has yet to have a "canonical" and opinionated approach to JSON-based API design, alongside the page rendering.
There are multiple JSON/API gems, but they come and go over the years, with inconsistent maintenance and development.
So, I will pick no fight with Basecamp about using server-side rendering for Hey, but I'll still be a bit disappointed that this hasn't become an opportunity to extend Rails support in another direction.
I could be mistaken, but I believe more recent versions of Rails have out-of-the-box tools for creating and maintaining JSON APIs. Many of the bootcamp grads I work with seem to have had experience building JSON APIs using a vanilla rails app.
https://github.com/rails/jbuilder
For a few years now, it has included the ability to focus strictly on APIs, leaving out the pieces you'd need for a full-blown browser-rendered app.