I was about to do the same post when I saw your's. So, same issue here. Why is the default value of "doc" used in logstash-output-elasticsearch ? (source: https://www.elastic.co/guide/en/logstash/6.2/plugins-outputs-elasticsearch.html#plugins-outputs-elasticsearch-document_type).
Moreover, that's been a while I removed all the document_type fields in my logstash configuration files but I still have this warning in my logfiles:
[2018-02-13T13:32:45,781][WARN ][logstash.outputs.elasticsearch] You are using a deprecated config setting "document_type" set in elasticsearch. Deprecated settings will continue to work, but are scheduled for removal from logstash in the future. Document types are being deprecated in Elasticsearch 6.0, and removed entirely in 7.0. You should avoid this feature If you have any questions about this, please visit the #logstash channel on freenode irc. blablablabla.....}
I use a 6.2.0 stack and the pipeline management feature of Kibana. I've absolutely no configuration in /etc/logstash and all the configurations stored in the pipeline management (in Elasticsearch) are "document_type"-free in Elasticsearch output.
Could we have any answer from the core-devs of Logstash ?
Have a nice day,