anchor.diff:  //Installation and Upgrade Guide
   
   //tag::notable-breaking-changes[]
  +[[id-field-data]]
  +.Aggregating and sorting on `_id` is disallowed by default.
  +[%collapsible]
  +====
  +*Details* +
  +Previously, it was possible to aggregate and sort on the built-in `_id` field
  +by loading an expensive data structure called fielddata. This was deprecated
  +in 7.6 and is now disallowed by default in 8.0.
  +
  +*Impact* +
  +Aggregating and sorting on `_id` should be avoided. As an alternative, the
  +`_id` field's contents can be duplicated into another field with docvalues
  +enabled (note that this does not apply to auto-generated IDs).
  +====
   //end::notable-breaking-changes[]
   
   .Search-related REST API endpoints containing mapping types have been removed.
  
positive.code: Change any use of `-1` as `from` parameter in request body or url parameters by either
  setting it to `0` or omitting it entirely. Requests containing negative values will
  return an error.
  ====
negative.code: GitHub or the 'discuss' forums.
  
  *Impact* +
  Discontinue use of the `sparse_vector` field data type. Requests containing
  a mapping for this field data type will return an error.
  ====
  
  