returning a 400 never prevented me from adding more info to the response

However you like, REST doesn’t dictate anything there. Just be consistent and use hypermedia.

JSON APIs almost never follow REST because they almost never use JSON as hypertext. Worse, no complete stable hypertext JSON standard exists. There’s JSON-HAL, but it lacks a way to represent resource templates (think HTML’s <form>).

Therefore, with JSON APIs ignoring one of the most basic idea behind REST, why would anyone expect them to follow another idea of REST - consistency?

REST is a deceptively simple concept. Any time you build an HTML website a human can navigate without consulting documentation, you’re doing it better than vast majority of swagger documented corporate APIs.

The argument probably goes something like " if you adhere strictly to REST the error codes are all you need" and then metadata can be sent in response headers.