elasticsearch/docs/reference/migration/migrate_6_0/rest.asciidoc

[[breaking_60_rest_changes]]
=== REST changes

==== Unquoted JSON

In previous versions of Elasticsearch, JSON documents were allowed to contain unquoted field names.
This feature was removed in the 5.x series, but a backwards-compatibility layer was added via the
system property `elasticsearch.json.allow_unquoted_field_names`. This backwards-compatibility layer
has been removed in Elasticsearch 6.0.0.