A new ingest processor plugin named ingest-h3 has been released. This plugin provides support for generating Uber H3 geo indexes from a given geo location point at given resolutions. Uber H3 is an hexagonal hierarchical geospatial indexing system, which partitions the space into an hexagonal grid (as opposed to rectangular grid with conventional geohashes).
It is worth noting that this plugin only allows to generate the indexes (simple keywords) within an ingest pipeline, but doesn't add any additional functionality to Elasticsearch to allow aggregating on them, yet.
For people who are interested, the plugin and some examples are available here: