So, this is actually about: Why I Tend Not To Use Content Negotiation (to serve both HTML and JSON data from the same endpoints).

The author also suggests:

> The alternative is to ... splitting your APIs. This means providing different paths (or sub-domains, or whatever) for your JSON API and your hypermedia (HTML) API.

I believe the alternative has been the norm actually. For example, many front-end frameworks encode UI states in URL, and it's not so sustainable to keep the alignment b/w UI states and data APIs in the long term.