AWS managed elastic search includes now a vector space knn index type (https://docs.aws.amazon.com/elasticsearch-service/latest/developerguide/knn.html) which is based on open distro elastic search (https://opendistro.github.io/for-elasticsearch/features/knn.html)
does it support working with sparse vectors?
I couldn't find anything in the docs, but the underlying nmslib library (https://github.com/nmslib/nmslib) does support sparse vector indices.
As of the moment this is not supported yet in Opendistro Elasticsearch. However, there is already a github issue raised: https://github.com/opendistro-for-elasticsearch/k-NN/issues/112