The dirty little secret of "RESTful" APIs is that everyone was pretty much generating bespoke responses to every request anyway out of data coming out of a datastore.

Nobody has static files sitting on a server anymore except for static auxiliary assets (CSS, scripts, fonts) or if you're actually running a website with static content, which is exceedingly rare. Everyone has some kind of request router that parses the URL and the body and figures out what to do next, makes a query to a backing database, then assembles and massages the response to make it look like the mediatype the client expects.