anchor.diff:  [[system-indices]]
   ==== System indices
   
  -{es} modules and plugins can store configuration and state information in internal _system indices_. 
  -You should not directly access or modify system indices 
  +{es} modules and plugins can store configuration and state information in internal _system indices_.
  +You should not directly access or modify system indices
   as they contain data essential to the operation of the system.
   
  -IMPORTANT: Direct access to system indices is deprecated and 
  +IMPORTANT: Direct access to system indices is deprecated and
   will no longer be allowed in the next major version.
   
   [discrete]
  
positive.code: [[api-compatibility]]
  === REST API version compatibility
  
  Major version upgrades often include a number of breaking changes 
  that impact how you interact with {es}. 
  While we recommend that you monitor the deprecation logs and 
  update applications before upgrading {es},
  having to coordinate the necessary changes can be an impediment to upgrading. 
  
  You can enable an existing application to function without modification after
  an upgrade by including API compatibility headers, which tell {es} you are still
  using the previous version of the REST API. Using these headers allows the 
  structure of requests and responses to remain the same; it does not guarantee
  the same behavior. 
  
  
  You set version compatibility on a per-request basis in the `Content-Type` and `Accept` headers. 
  Setting `compatible-with` to the same major version as 
  the version you're running has no impact, 
  but ensures that the request will still work after {es} is upgraded. 
  
  To tell {es} 8.0 you are using the 7.x request and response format, 
  set `compatible-with=7`:
  
  [source,sh]
  
negative.code: by default. To match hidden data streams and indices using a wildcard
  expression, you must specify the `expand_wildcards` query parameter.
  
  Alternatively, querying an index pattern starting with a dot, such as
  `.watcher_hist*`, will match hidden indices by default. This is intended to
  mirror Unix file-globbing behavior and provide a smoother transition path to
  hidden indices.
  
  You can create hidden data streams by setting `data_stream.hidden` to `true` in
  the stream's matching <<indices-put-template,index template>>. You can hide
  indices using the <<index-hidden,`index.hidden`>> index setting.
  