https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...
https://developer.chrome.com/extensions/settings_override
The fact that it doesn't have "open" in its name doesn't actually make it any less open or interoperable. This is about standardizing on having a single and modern way to define search engines, nothing more and nothing less.
I lump this in with the kinds of decisions like removing RSS support from browsers, browsers (hello Chrome) not supporting MathML[1], and a zillion other "death by a thousand cuts" things that impede the Open Web.
[1]: Yes, I know that there is an initiative underway to re-add MathML support to Chrome. I'll remain somewhat skeptical until it is actually shipping and works.
If they were just removing OpenSearch in xpi on AMO, that'd be fine, but the post says they intend to remove OpenSearch as a whole.
...which have nothing to do with the Open Web, mentioned in the parent reply. Both require participating in, and consequently adhering to the terms of, stores operated by third parties.
Which, in turn, has nothing to do with the origin article. Read it again, it is only about how search engines are defined inside add-ons.