Right now, Phabricator is very difficult to integrate into other search tools because of its use of PhabricatorSearchQuery (random 12-byte identifiers for all search queries). It would be great if there were either an OpenSearch plugin provider, or at least a GET-based non-CSRF-protected endpoint for creating a SearchQuery.
Since OpenSearch looks at least moderately complex and no one else has asked for it it's probably going to be hard to prioritize.
Non-CSRF GET endpoint we can do fairly easily, though. We could also expose search over the Conduit API easily, although your options for plugging that into other things right now are kind of indirect/hard too (we could hook it up to arc and provide arc search <term> / arc search --json <term>, but that might also be too many levels removed to be compelling, and incurs some extra overhead for the auth handshake).
Making URLs shareable/manageable, working for longer-than-GET-URL-length queries, keeping track of what users search for so we can tune search to help them find it, allowing queries to be migrated across API changes, allowing queries to be saved/named (not currently in general search, but implemented in Maniphest). Slightly more discussion in or around T2625, which I want to bring everywhere once it is implemented.
We have GET-accessible queries now, here's an example:
This task is about a year old now, though, and we haven't seen other requests for OpenSearch support in that time. Without more demand, the complexity of supporting OpenSearch is hard to justify.
There's demand for OpenSearch support at our installation; I have a patch that implements non-typeahead OpenSearch built and working, but I figured I'd bump the ticket in case anyone else was interested.