Elastic plans to deprecate document types, and instead use a new “_join" (metadata?) field for modelling parent-child relationships. ES 6.x will still support old indices with multiple types, but allow only one type for new indices. With ES 7.x, they plan to drop the document types altogether.
https://www.elastic.co/blog/index-type-parent-child-join-now-future-in-elasticsearch
Customers need to consider how to model their data, to avoid compatibility issues when upgrading.