The changes related to mapping are as follows:

• The original indexing meta field, _all, which indexed the values of all fields, has been eliminated. 
• The original indexing meta field, _uid, which combined _type and _id, has been eliminated.
• The original default mapping meta field, _default_, which was used as the base mapping for any new mapping type, has been eliminated. 
• For search and highlighting purposes, the index_options parameter  controls which information has been added to the inverted index. However, it no longer supports numeric fields.
• The max limit of nested JSON objects within a single document across all nested fields is 10000.
• In the past, specifying that the update_all_types parameter update the mappings would update all fields with the same name of all _type in the same index. It has been eliminated.
• The classic similarity feature, which is based on the TF/IDF to define how matching documents are scored, has been eliminated since it is no longer supported by Lucene.
• The error for providing unknown similarity parameters in the request will fail with exception. 
• The geo_shape datatypes in the indexing strategy now defaults to using a vector-indexing approach based on Lucene's new LatLonShape field type.  
• Most options of the geo_shape mapping will be eliminated in a future version. They are tree, precision, tree_levels, strategy, distance_error_pct, and points_only. 
• The max limit of completion context is 10. A deprecation warning will be logged if the setting exceeds.
• The default value of include_type_name has changed from true to false.