Previously templates expressed the indices that they should match using a glob style pattern in the template field. They should now use the index_patterns field instead. As the name implies you can define multiple glob style patterns in an array but for convenience defining a single pattern as a bare string is also supported. So both of these examples are valid:

PUT _template/template_1
{
  "index_patterns": ["te*", "bar*"],
  "settings": {
    "number_of_shards": 1
  }
}
PUT _template/template_2
{
  "index_patterns": "te*",
  "settings": {
    "number_of_shards": 1
  }
}

Shadow replicas don’t see enough usage, and have been removed. This includes the following settings:

The default value of the allow_no_indices option for the Open/Close index API has been changed from false to true so it is aligned with the behaviour of the Delete index API. As a result, Open/Close index API don’t return an error by default when a provided wildcard expression doesn’t match any closed/open index.

Delete a document from non-existing index has been modified to not create the index. However if an external versioning is used the index will be created and the document will be marked for deletion.

The index parameter in the update-aliases, put-alias, and delete-alias APIs no longer accepts alias names. Instead, it accepts only index names (or wildcards which will expand to matching indices).

The index parameter in the delete index API no longer accepts alias names. Instead, it accepts only index names (or wildcards which will expand to matching indices).

Omitting the + has the same effect as specifying it, hence support for + has been removed in index expressions.

Translog files are now kept for up to 12 hours (by default), with a maximum size of 512mb (default), and are no longer deleted on flush. This is to increase the chance of doing an operation based recovery when bringing up replicas up to speed.

To safeguard against creating too many index terms, the difference between max_ngram and min_ngram in NGramTokenFilter and NGramTokenizer has been limited to 1. This default limit can be changed with the index setting index.max_ngram_diff. Note that if the limit is exceeded a deprecation warning is logged.

To safeguard against creating too many tokens, the difference between max_shingle_size and min_shingle_size in ShingleTokenFilter has been limited to 3. This default limit can be changed with the index setting index.max_shingle_diff. Note that if the limit is exceeded a deprecation warning will be logged.