Hm.... checking the 5.5 branch I clearly see _type being hardcoded to docs.
The _type field is set here: https://github.com/elastic/beats/blob/5.5/libbeat/outputs/elasticsearch/client.go#L325
with eventType being hardcoded to "doc": https://github.com/elastic/beats/blob/5.5/libbeat/outputs/elasticsearch/client.go#L96
Are you using the Elasticsearch output or Logstash?