The latest version of Curator should be version 6.5 and even if there are no changes, it should be updated with the version numbers of everything else.
If it just so happens to be compatible with more than the other products, that's fine. It's confusing that everything else follows a rigid compatibility matrix, but curator does not.
Curator does not follow the general versioning because:
It's not a product, like Logstash, Beats, Kibana, etc.
Curator has different cross-version compatibility from everything else
It's developed and maintained by one person
It is being supplanted (at least, in large part) by Index Lifecycle Management, which is slated to arrive in Elasticsearch 6.6.0.
Because of the drive to build in Curator-like functionality, which is part of a product, Curator will continue to exist, but as a side project, and will likely continue to be developed and supported by one person.
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