You can override the default mappings for all indices and all types by specifying a _default_ type mapping in an index template which matches all indices.

For example, to disable the _all field by default for all types in all new indices, you could create the following index template:

PUT _template/disable_all_field
{
  "order": 0,
  "template": "*", 
  "mappings": {
    "_default_": { 
      "_all": { 
        "enabled": false
      }
    }
  }
}