Remove unfreeze REST endpoint (elastic#119227)

This adds a sentence to `redirects.asciidoc` explaining what frozen
indices were - otherwise, everything will point to the message about
the unfreeze API having gone away, which is not very helpful. Some
cross-references are updated to point to this rather than to the
notice about the removal of the unfreeze API.

ES-9736 #comment Removed `_unfreeze` REST endpoint in elastic#119227

Author: PeteGillinElastic

docs/changelog/119227.yaml

@@ -0,0 +1,13 @@
+pr: 119227
+summary: Remove unfreeze REST endpoint
+area: Indices APIs
+type: breaking
+issues: []
+breaking:
+  title: Remove unfreeze REST endpoint
+  area: REST API
+  details: >-
+    The `/{index}/_unfreeze` REST endpoint is no longer supported. This API was deprecated, and the corresponding
+    `/{index}/_freeze` endpoint was removed in 8.0.
+  impact: None, since it is not possible to have a frozen index in a version which is readable by Elasticsearch 9.0
+  notable: false

docs/reference/indices.asciidoc

@@ -24,7 +24,6 @@ index settings, aliases, mappings, and index templates.
 * <<indices-split-index>>
 * <<indices-clone-index>>
 * <<indices-rollover-index>>
-* <<unfreeze-index-api>>
 * <<indices-resolve-index-api>>
 * <<indices-resolve-cluster-api>>
 * <<indices-downsample-data-stream>>
@@ -143,6 +142,5 @@ include::indices/shrink-index.asciidoc[]
 include::indices/simulate-index.asciidoc[]
 include::indices/simulate-template.asciidoc[]
 include::indices/split-index.asciidoc[]
-include::indices/apis/unfreeze.asciidoc[]
 include::indices/update-settings.asciidoc[]
 include::indices/put-mapping.asciidoc[]

docs/reference/indices/apis/unfreeze.asciidoc

@@ -43,7 +43,7 @@ For more information on managing indices, refer to <<indices, Index APIs>>.
 
 * To filter the list of indices, use the search bar or click a badge.
 Badges indicate if an index is a <<ccr-put-follow,follower index>>, a
-<<rollup-get-rollup-index-caps,rollup index>>, or <<unfreeze-index-api,frozen>>.
+<<rollup-get-rollup-index-caps,rollup index>>, or <<frozen-indices,frozen>>.
 
 * To drill down into the index
 <<mapping,mappings>>, <<index-modules-settings,settings>>, and statistics,

docs/reference/indices/index-mgmt.asciidoc

@@ -156,10 +156,16 @@ See <<restore-entire-cluster>>.
 The freeze index API was removed in 8.0.
 // tag::frozen-removal-explanation[]
 Frozen indices are no longer useful due to
-https://www.elastic.co/blog/significantly-decrease-your-elasticsearch-heap-memory-usage[recent
-improvements in heap memory usage].
+https://www.elastic.co/blog/significantly-decrease-your-elasticsearch-heap-memory-usage[improvements
+in heap memory usage].
 // end::frozen-removal-explanation[]
 
+[role="exclude",id="unfreeze-index-api"]
+=== Unfreeze index API
+
+The unfreeze index API was removed in 9.0.
+include::redirects.asciidoc[tag=frozen-removal-explanation]
+
 [role="exclude",id="ilm-freeze"]
 === Freeze {ilm-init} action
 
@@ -1749,8 +1755,10 @@ See <<search-terms-enum>>.
 === Frozen indices
 
 // tag::frozen-index-redirect[]
-
-For API documentation, see <<unfreeze-index-api>>.
+Older versions of {es} provided the option to reduce the amount of data kept in memory for an index, at the expense of
+increasing search latency. This was known as 'freezing' the index.
+include::redirects.asciidoc[tag=frozen-removal-explanation]
+The freeze index API was removed in 8.0, and the unfreeze index API was removed in 9.0.
 // end::frozen-index-redirect[]
 
 [role="exclude",id="best_practices"]

docs/reference/redirects.asciidoc

@@ -100,7 +100,7 @@ requires the keyword `LIKE` for SQL `LIKE` pattern.
 [[sql-index-frozen]]
 === Frozen Indices
 
-By default, {es-sql} doesn't search <<unfreeze-index-api,frozen indices>>. To
+By default, {es-sql} doesn't search <<frozen-indices,frozen indices>>. To
 search frozen indices, use one of the following features:
 
 dedicated configuration parameter::

docs/reference/sql/language/indices.asciidoc

@@ -8,36 +8,22 @@
 
 import org.elasticsearch.action.ActionRequest;
 import org.elasticsearch.action.ActionResponse;
-import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
-import org.elasticsearch.cluster.node.DiscoveryNodes;
-import org.elasticsearch.common.io.stream.NamedWriteableRegistry;
-import org.elasticsearch.common.settings.ClusterSettings;
-import org.elasticsearch.common.settings.IndexScopedSettings;
 import org.elasticsearch.common.settings.Setting;
-import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.common.settings.SettingsFilter;
-import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.engine.EngineFactory;
 import org.elasticsearch.index.engine.frozen.FrozenEngine;
 import org.elasticsearch.plugins.ActionPlugin;
 import org.elasticsearch.plugins.EnginePlugin;
 import org.elasticsearch.plugins.Plugin;
-import org.elasticsearch.rest.RestController;
-import org.elasticsearch.rest.RestHandler;
 import org.elasticsearch.xpack.core.action.XPackInfoFeatureAction;
 import org.elasticsearch.xpack.core.action.XPackUsageFeatureAction;
 import org.elasticsearch.xpack.core.frozen.action.FreezeIndexAction;
 import org.elasticsearch.xpack.frozen.action.TransportFreezeIndexAction;
-import org.elasticsearch.xpack.frozen.rest.action.RestFreezeIndexAction;
 
 import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
-import java.util.function.Predicate;
-import java.util.function.Supplier;
 
 public class FrozenIndices extends Plugin implements ActionPlugin, EnginePlugin {
 
@@ -63,19 +49,4 @@ public List<Setting<?>> getSettings() {
         actions.add(new ActionHandler<>(FreezeIndexAction.INSTANCE, TransportFreezeIndexAction.class));
         return actions;
     }
-
-    @Override
-    public List<RestHandler> getRestHandlers(
-        Settings settings,
-        NamedWriteableRegistry namedWriteableRegistry,
-        RestController restController,
-        ClusterSettings clusterSettings,
-        IndexScopedSettings indexScopedSettings,
-        SettingsFilter settingsFilter,
-        IndexNameExpressionResolver indexNameExpressionResolver,
-        Supplier<DiscoveryNodes> nodesInCluster,
-        Predicate<NodeFeature> clusterSupportsFeature
-    ) {
-        return Collections.singletonList(new RestFreezeIndexAction());
-    }
 }