Earlier quoted context omitted.
AFAIK when it comes to nearest-neighbor searches Elasticsearch is catching up with OpenDistro k-NN, and not the other way around.
Perhaps, but the point about divergence stands! Elastic won’t be copying the OpenDistro KNN API.
From the linked issue, it seemed that they were originally planning to develop this as a proprietary feature of Elasticsearch, without contributing the functionality to Apache Lucene, but then changed direction when the Apache Lucene developers (some of which are currently employed to do such work by Amazon) started to build its approximate nearest neighbor (ANN) vector search capabilities. [1]
It's great to see folks that work for Elastic collaborating and building on what is in Apache Lucene to extend the utility of ANN with Hierarchical Navigable Small World Graphs (HNSW) [2]! From this, I think it should be possible to implement an Open Source version of the functionality with a compatible API, if that is something that OpenSearch users seek.