Firefox is getting rid of, or has already got rid off, OpenSearch integrations[1].

What you're describing is a bookmark substitution search feature that's independent of OpenSearch, and Mozilla calls the feature keyword searches[2].

[1] https://en.wikipedia.org/wiki/OpenSearch#Support

[2] http://kb.mozillazine.org/Using_keyword_searches

My understanding is that Firefox still uses the OpenSearch metadata shallowly as the easiest way to create the "keyword search bookmarks" rather than writing a bookmark by hand (though your second link was what I was looking for on how to do that and it seems quite easy/convenient to do yourself if you had to). I don't believe that shallow auto-bookmark discovery is/will be dropped, they dropped a more complicated add-on/extension-based approach that supported more OpenSearch features.

Thanks for taking the time to explain the change, it looks like you're correct.