I can't say I've seen that many examples of APIs that use them richly. You see a little bit of in in specs that require something be done a certain way (WHEP in particular requires you send ICE information in ice-server link relations). Part of it probably stems from the fact that a lot of the tooling was built on top of OpenAPI (which had discriminators built in and so people used that for implementing "sum types"), etc, and part of that comes from the fact that a lot of HTTP-based APIs basically only take the content portion of the response, ignoring the headers basically entirely (outside of maybe redirects).

Relatively speaking I think there aren't that many people that know that it is even really a possibility, let alone use it.

(You even see discussions on even whether the status codes are to be used at all, when they were really purpose-built to be generalizable.)

This partially is self-reinforcing: the server libraries don't do anything with media types generally outside of maybe a "FileResponse"-ish class or function and the client libraries don't tend to implement it because it's not supported.

Part of it too may be that interoperability isn't necessarily an underlying goal.

> one other thought, I'm starting to see how the semantic web fits into this vision. Too bad it was too brittle in practice (though the SOLID folks are doing some exciting stuff).

Indeed! There's a lot of similar ideas with this and you definitely have things you can fit in there. :)