Restore changes that shouldn't be ported

Author: alexey-ivanov-es

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

@@ -170,9 +170,7 @@ public DataStream(
         this.name = name;
         this.generation = generation;
         this.metadata = metadata;
-        // The following assert is commented out, because system data streams created before 8.1 are not hidden,
-        // but should be updated to hidden by 8.18/8.19 (SystemIndexMetadataUpgradeService)
-        // assert system == false || hidden; // system indices must be hidden
+        assert system == false || hidden; // system indices must be hidden
         this.hidden = hidden;
         this.replicated = replicated;
         this.timeProvider = timeProvider;

x-pack/qa/rolling-upgrade/src/test/resources/rest-api-spec/test/upgraded_cluster/80_transform_jobs_crud.yml

@@ -97,9 +97,6 @@ setup:
 ---
 "Get start, stop mixed cluster batch transform":
   # Simple and complex Mixed cluster transforms
-  - skip:
-      awaits_fix: "https://github.com/elastic/ml-team/issues/1522"
-      reason: "Transform system index migration is broken"
   - do:
       transform.get_transform:
         transform_id: "mixed-simple-transform"
@@ -228,9 +225,6 @@ setup:
         transform_id: "old-simple-continuous-transform"
 ---
 "Test GET, mixed continuous transforms":
-  - skip:
-      awaits_fix: "https://github.com/elastic/ml-team/issues/1522"
-      reason: "Transform system index migration is broken"
   - do:
       transform.get_transform:
         transform_id: "mixed-simple-continuous-transform"
@@ -295,9 +289,6 @@ setup:
 
 ---
 "Test index mappings for latest internal index and audit index":
-  - skip:
-      awaits_fix: "https://github.com/elastic/ml-team/issues/1522"
-      reason: "Transform system index migration is broken"
   - skip:
       features: warnings
   - requires: