fix setting registration

Author: tlrx

server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java

@@ -2514,7 +2514,9 @@ IndexMetadata build(boolean repair) {
             final IndexMode indexMode = indexModeString != null ? IndexMode.fromString(indexModeString.toLowerCase(Locale.ROOT)) : null;
             final boolean isTsdb = indexMode == IndexMode.TIME_SERIES;
             boolean useTimeSeriesSyntheticId = false;
-            if (isTsdb && indexCreatedVersion.onOrAfter(IndexVersions.TIME_SERIES_USE_SYNTHETIC_ID)) {
+            if (isTsdb
+                && IndexSettings.TSDB_SYNTHETIC_ID_FEATURE_FLAG
+                && indexCreatedVersion.onOrAfter(IndexVersions.TIME_SERIES_USE_SYNTHETIC_ID)) {
                 var setting = settings.get(IndexSettings.USE_SYNTHETIC_ID.getKey());
                 if (setting != null && setting.equalsIgnoreCase(Boolean.TRUE.toString())) {
                     assert IndexSettings.TSDB_SYNTHETIC_ID_FEATURE_FLAG;

server/src/main/java/org/elasticsearch/index/IndexSettings.java

@@ -1182,7 +1182,7 @@ public IndexSettings(final IndexMetadata indexMetadata, final Settings nodeSetti
             && scopedSettings.get(RECOVERY_USE_SYNTHETIC_SOURCE_SETTING);
         useDocValuesSkipper = DOC_VALUES_SKIPPER && scopedSettings.get(USE_DOC_VALUES_SKIPPER);
         seqNoIndexOptions = scopedSettings.get(SEQ_NO_INDEX_OPTIONS_SETTING);
-        final var useSyntheticId = scopedSettings.get(USE_SYNTHETIC_ID);
+        final var useSyntheticId = IndexSettings.TSDB_SYNTHETIC_ID_FEATURE_FLAG && scopedSettings.get(USE_SYNTHETIC_ID);
         if (indexMetadata.useTimeSeriesSyntheticId() != useSyntheticId) {
             assert false;
             throw new IllegalArgumentException(