Previously a document could be indexed as itself, or wrapped in an outer object which specified the type name:

PUT /my_index/my_type/1
{
  "my_type": {
     ... doc fields ...
  }
}

This led to some ambiguity when a document also included a field with the same name as the type. We no longer accept the outer type wrapper, but this behaviour can be reenabled on an index-by-index basis with the setting: index.mapping.allow_type_wrapper.