Elasticsearch used to be available under an Apache 2.0 license. It has now switched to a proprietary license in which the source code is published, but reuse is by permission.
On the mundane front, this means license information in extension-cirrussearch documentation is out-of date (I think).
Since Elasticsearch is no longer freely-licenced in a way that allows reuse, it is not really compatible with Wikimedia's reuse policies. There is a (still-compatible) fork called Opensearch.
The Opensearch software is mostly being developed by Amazon, which is a conflict of interest (I realize I'm writing this on Facebook-supported software). Other reusably-licensed search software includes YaCy and Gigablast. I assume the WMF would, in principle, be willing to give a grant for the development of suitable software.
Comments, suggestions?