[+opensearch@]
Hi Igor! Good question. I think the namespace identifier not returning content when resolved as a URL is unfortunate, but not strictly a bug, at least by XML spec. I wrote a little bit about that before:
As for JSON, nothing ever got formalized, and I can't say I recall ever seeing a particular leading proposal emerge.
At this point, I'd personally be encouraging the community to start thinking about how to re-invent OpenSearch for the modern web using something like
schema.org as the basis, and maybe looking at JSON-LD as a possible serialization (I'm not an expert here). Not that the underlying concepts are bad, they just are a little out of date with how the web evolved.
That's perhaps out of scope for your actual question, but bringing federated search, both structured results and search engine discovery, into an HTML5 and mobile-first world would be a very interesting exercise.
-DeWitt