The _all meta field is now disabled by default
On new mappings, the _all meta field that contains a copy of the text from each field is now disabled by default. The query_string and simple_query_string queries that previously used _all to search will now check if _all is enabled/disabled and switch to executing the query across all fields if _all is disabled. _all can no longer be configured for indices created with Elasticsearch version 6.0 or later.
Multiple mapping types are not supported in indices created in 6.0
The ability to have multiple mapping types per index has been removed
in 6.0. New indices will be restricted to a single type. This is the
first step in the plan to remove mapping types altogether. Indices
created in 5.x will continue to support multiple mapping types.
ElasticSearch docs regarding changes to the _all meta field in v6 for reference: https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-6.0.html#_the_literal__all_literal_meta_field_is_now_disabled_by_default
Magento Commerce version 2.3.x supports the following Elasticsearch versions:
- Elasticsearch 5.2.x
- Elasticsearch 2.x
- Magento 2.3 uses Elasticsearch PHP client version 5.2. (Before
version 2.3, Magento used PHP client version 5.1.)
I hope this will help