[DOCS] Add 8.0 release highlight for system indices

docs/reference/release-notes/highlights.asciidoc

@@ -58,6 +58,28 @@ Need a new enrollment token? Use the
 {ref}/create-enrollment-token.html[`elasticsearch-create-enrollment-token`] 
 tool to create enrollment tokens for {es} nodes and {kib} instances.
 
+[discrete]
+=== Better protection for system indices
+
+System indices store configurations and internal data for Elastic features.
+Generally, system indices are reserved only for internal use by these features.
+While possible, directly accessing or changing a system index can cause
+instability and other issues.
+
+In 8.0, we've made several changes to protect system indices from direct access.
+To access a system index, you must now have the
+{ref}/defining-roles.html#roles-indices-priv[`allow_restricted_indices`]
+permission set to `true`.
+
+The `superuser` role also no longer gives write access to system indices. As a
+result, the built-in `elastic` superuser can't change system indices by
+default.
+
+If available, use {kib} or the associated {es} APIs to manage data for a feature
+rather than accessing a system index. If you attempt to directly access a system index,
+{es} will return a warning in the header of API responses and in the deprecation
+logs.
+
 [discrete]
 === Storage savings for `keyword`, `match_only_text`, and `text` fields