[DOCS] Add deprecation notice for system indices (elastic#83688)

* [DOCS] Add deprecation notice for system indices

* Fix spacing issue

Author: Adam Locke

docs/reference/migration/migrate_8_0/index-setting-changes.asciidoc

@@ -6,6 +6,24 @@
 //Installation and Upgrade Guide
 
 //tag::notable-breaking-changes[]
+[[deprecation-system-indices]]
+.Direct access to system indices is deprecated.
+[%collapsible]
+====
+*Details* +
+Directly accessing system indices is deprecated, and may be prevented in a
+future version. If you must access a system index, create a security role with
+an index permission that targets the specific index and set the
+`allow_restricted_indices` permission to `true`. Refer to 
+{ref}/defining-roles.html#roles-indices-priv[indices privileges] for
+information on adding this permission to an index privilege.
+
+*Impact* +
+Accessing system indices directly results in warnings in the header of API
+responses. If available, use {kib} or the associated feature's {es} APIs to
+manage the data that you want to access.
+====
+
 [[deprecate-max-merge-at-once-explicit-setting]]
 .`index.merge.policy.max_merge_at_once_explicit` is deprecated and has no effect.
 [%collapsible]