I have started using Elasticsearch 6.5 version with parent-child actions (mapping, indexing, updating, deleting). In case of delete, documentation says ROUTING value is required. In a reality ROUTING value is not required and so it works! Because parent and child are in a same index (from ES >=6.0). So, is documentation wrongly typed or is it ES bug?
Documentation page: https://www.elastic.co/guide/en/elasticsearch/reference/current/parent-join.html:
Only one join field mapping is allowed per index.
Parent and child documents must be indexed on the same shard. This means that the same routing value needs to be provided when getting, deleting, or updating a child document.
An element can have multiple children but only one parent.
It is possible to add a new relation to an existing join field.
It is also possible to add a child to an existing element but only if the element is already a parent.