Add frozen index deprecation check.

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/DeprecationChecks.java

@@ -41,7 +41,8 @@ private DeprecationChecks() {
         IndexDeprecationChecks::oldIndicesCheck,
         IndexDeprecationChecks::translogRetentionSettingCheck,
         IndexDeprecationChecks::checkIndexDataPath,
-        IndexDeprecationChecks::storeTypeSettingCheck
+        IndexDeprecationChecks::storeTypeSettingCheck,
+        IndexDeprecationChecks::frozenIndexSettingCheck
     );
 
     /**

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecks.java

@@ -11,6 +11,7 @@
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.index.IndexModule;
 import org.elasticsearch.index.IndexSettings;
+import org.elasticsearch.index.engine.frozen.FrozenEngine;
 import org.elasticsearch.xpack.core.deprecation.DeprecationIssue;
 
 import java.util.Locale;
@@ -73,4 +74,21 @@ static DeprecationIssue storeTypeSettingCheck(IndexMetadata indexMetadata) {
         }
         return null;
     }
+
+    static DeprecationIssue frozenIndexSettingCheck(IndexMetadata indexMetadata) {
+        Boolean isIndexFrozen = FrozenEngine.INDEX_FROZEN.get(indexMetadata.getSettings());
+        if (Boolean.TRUE.equals(isIndexFrozen)) {
+            String indexName = indexMetadata.getIndex().getName();
+            return new DeprecationIssue(
+                DeprecationIssue.Level.WARNING,
+                "index [" + indexName +
+                    "] is a frozen index. The frozen indices feature is deprecated and will be removed in a future version",
+                "https://www.elastic.co/guide/en/elasticsearch/reference/master/frozen-indices.html",
+                "Frozen indices no longer offer any advantages. Consider cold or frozen tiers in place of frozen indices.",
+                false,
+                null
+            );
+        }
+        return null;
+    }
 }

x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/IndexDeprecationChecksTests.java

@@ -12,6 +12,7 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.index.IndexModule;
 import org.elasticsearch.index.IndexSettings;
+import org.elasticsearch.index.engine.frozen.FrozenEngine;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.xpack.core.deprecation.DeprecationIssue;
 
@@ -96,4 +97,20 @@ public void testSimpleFSSetting() {
                     "as it offers superior or equivalent performance to [simplefs].", false, null)
         ));
     }
+
+    public void testFrozenIndex() {
+        Settings.Builder settings = settings(Version.CURRENT);
+        settings.put(FrozenEngine.INDEX_FROZEN.getKey(), true);
+        IndexMetadata indexMetadata = IndexMetadata.builder("test").settings(settings).numberOfShards(1).numberOfReplicas(0).build();
+        List<DeprecationIssue> issues = DeprecationChecks.filterChecks(INDEX_SETTINGS_CHECKS, c -> c.apply(indexMetadata));
+        assertThat(issues, contains(
+            new DeprecationIssue(DeprecationIssue.Level.WARNING,
+                "index [test] is a frozen index. The frozen indices feature is deprecated and will be removed in a future version",
+                "https://www.elastic.co/guide/en/elasticsearch/reference/master/frozen-indices.html",
+                "Frozen indices no longer offer any advantages. Consider cold or frozen tiers in place of frozen indices.",
+                false,
+                null
+            )
+        ));
+    }
 }