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.
Elasticsearch is still open source...just not in a license that Amazon can take advantage of.
Specifically, this feature https://github.com/elastic/elasticsearch/issues/42326#issuec... will be a big change for Elasticsearch. OpenSearch might try to mimic the API, but implementation details here will matter a lot, since this type of search is really picky when it comes to performance/recall balance. OpenDistro has already been working on their own version: https://opendistro.github.io/for-elasticsearch/features/knn.... ...so will they switch their API? Perhaps - but the results are going to be very different.