The deprecation API is to be used to retrieve information about different cluster, node, and index level settings that use deprecated features that will be removed or changed in the next major version.
GET /_migration/deprecations
GET /<index_name>/_migration/deprecations
index_name
To see the list of offenders in your cluster, submit a GET request to the
_migration/deprecations endpoint:
GET /_migration/deprecations
Example response:
{
  "cluster_settings" : [
    {
      "level" : "critical",
      "message" : "Cluster name cannot contain ':'",
      "url" : "https://www.elastic.co/guide/en/elasticsearch/reference/7.0/breaking-changes-7.0.html#_literal_literal_is_no_longer_allowed_in_cluster_name",
      "details" : "This cluster is named [mycompany:logging], which contains the illegal character ':'."
    }
  ],
  "node_settings" : [ ],
  "index_settings" : {
    "logs:apache" : [
      {
        "level" : "warning",
        "message" : "Index name cannot contain ':'",
        "url" : "https://www.elastic.co/guide/en/elasticsearch/reference/7.0/breaking-changes-7.0.html#_literal_literal_is_no_longer_allowed_in_index_name",
        "details" : "This index is named [logs:apache], which contains the illegal character ':'."
      }
    ]
  },
  "ml_settings" : [ ]
}The response breaks down all the specific forward-incompatible settings that you should resolve before upgrading your cluster. Any offending settings are represented as a deprecation warning.
The following is an example deprecation warning:
{
  "level" : "warning",
  "message" : "This is the generic descriptive message of the breaking change",
  "url" : "https://www.elastic.co/guide/en/elasticsearch/reference/6.0/breaking_60_indices_changes.html",
  "details" : "more information, like which nodes, indices, or settings are to blame"
}As is shown, there is a level property that describes the significance of the
issue.
| warning | You can upgrade directly, but you are using deprecated functionality which will not be available or behave differently in the next major version. | 
| critical | You cannot upgrade without fixing this problem. | 
The message property and the optional details property provide descriptive
information about the deprecation warning. The url property provides a link to
the Breaking Changes Documentation, where you can find more information about
this change.
Any cluster-level deprecation warnings can be found under the cluster_settings
key. Similarly, any node-level warnings are found under node_settings. Since
only a select subset of your nodes might incorporate these settings, it is
important to read the details section for more information about which nodes
are affected. Index warnings are sectioned off per index and can be filtered
using an index-pattern in the query. Machine Learning related deprecation
warnings can be found under the ml_settings key.
The following example request shows only index-level deprecations of all
logstash-* indices:
GET /logstash-*/_migration/deprecations