There seems to be a lot of confusion in the above thread; hopefully I'm writing in the correct place instead of opening a new issue - but I, too, noted the lack of OpenSearch support.
My personal use-case is setting up Flashbang, which I use across all of my browsers for consistent bang-search (usually through Kagi, which I'm a convert to, thanks entirely to Orion, btw! 😉
In FireFox, after literally just clicking that link (there's no further UI or interaction from the user, OpenSearch is zero-conf by design), this option permanently appears in the menubar dropdown for search-tool-selection:
Furthermore (slightly annoyingly to me, only after issuing a search with that dropdown, which I otherwise never use), it shows up in the FireFox preferences as a "Default search engine" option:
This flow is general for basically any website on the modern internet that has a searchfield; I find it to be quite widely adopted. I may be missing it, but I didn't see anywhere in the interface that flashbang appeared after loading their page.
If you want a super-lightweight UI, I feel the most minimal possible implementation would simply be automatically adding OpenSearch-declaring pages into the "Custom search engines" interface as soon as you recognize them during browsing:
