String datatype

The string field is unsupported for indexes created in 5.x in favor of the text and keyword fields. Attempting to create a string field in an index created in 5.x will cause Elasticsearch to attempt to upgrade the string into the appropriate text or keyword field. It will return an HTTP Warning header telling you that string is deprecated. This upgrade process isn’t always perfect because there are some combinations of features that are supported by string but not text or keyword. For that reason it is better to use text or keyword.

Indexes imported from 2.x only support string and not text or keyword. To ease the migration from 2.x Elasticsearch will downgrade text and keyword mappings applied to indexes imported from 2.x into string. While long lived indexes will eventually need to be recreated against 5.x before eventually upgrading to 6.x, this downgrading smooths the process before you find time for it.