diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index d4aa36a7881..af02dcbf246 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -195,6 +195,9 @@ Improvements
 
 * GITHUB#13041: TokenizedPhraseQueryNode code cleanup (Dmitry Cherniachenko)
 
+* GITHUB#13087: Changed `static final Set` constants to be immutable. Among others it affected
+  ScandinavianNormalizer.ALL_FOLDINGS set with public access. (Dmitry Cherniachenko)
+
 Optimizations
 ---------------------
 
diff --git a/lucene/analysis/common/src/java/org/apache/lucene/analysis/miscellaneous/ScandinavianNormalizer.java b/lucene/analysis/common/src/java/org/apache/lucene/analysis/miscellaneous/ScandinavianNormalizer.java
index 50187264a78..6654929d559 100644
--- a/lucene/analysis/common/src/java/org/apache/lucene/analysis/miscellaneous/ScandinavianNormalizer.java
+++ b/lucene/analysis/common/src/java/org/apache/lucene/analysis/miscellaneous/ScandinavianNormalizer.java
@@ -16,6 +16,7 @@
  */
 package org.apache.lucene.analysis.miscellaneous;
 
+import java.util.Collections;
 import java.util.EnumSet;
 import java.util.Set;
 import org.apache.lucene.analysis.util.StemmerUtil;
@@ -51,7 +52,8 @@ public final class ScandinavianNormalizer {
 
   private final Set<Foldings> foldings;
 
-  public static final Set<Foldings> ALL_FOLDINGS = EnumSet.allOf(Foldings.class);
+  public static final Set<Foldings> ALL_FOLDINGS =
+      Collections.unmodifiableSet(EnumSet.allOf(Foldings.class));
 
   static final char AA = '\u00C5'; // Å
   static final char aa = '\u00E5'; // å
diff --git a/lucene/analysis/common/src/test/org/apache/lucene/analysis/core/TestFactories.java b/lucene/analysis/common/src/test/org/apache/lucene/analysis/core/TestFactories.java
index 5ae7b111024..11c7cacef30 100644
--- a/lucene/analysis/common/src/test/org/apache/lucene/analysis/core/TestFactories.java
+++ b/lucene/analysis/common/src/test/org/apache/lucene/analysis/core/TestFactories.java
@@ -20,9 +20,7 @@ import java.io.IOException;
 import java.io.Reader;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
-import java.util.Arrays;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 import org.apache.lucene.analysis.AbstractAnalysisFactory;
@@ -55,10 +53,8 @@ public class TestFactories extends BaseTokenStreamTestCase {
 
   /** Factories that are excluded from testing it with random data */
   private static final Set<Class<? extends AbstractAnalysisFactory>> EXCLUDE_FACTORIES_RANDOM_DATA =
-      new HashSet<>(
-          Arrays.asList(
-              DelimitedTermFrequencyTokenFilterFactory.class,
-              DelimitedBoostTokenFilterFactory.class));
+      Set.of(
+          DelimitedTermFrequencyTokenFilterFactory.class, DelimitedBoostTokenFilterFactory.class);
 
   public void test() throws IOException {
     for (String tokenizer : TokenizerFactory.availableTokenizers()) {
diff --git a/lucene/analysis/kuromoji/src/test/org/apache/lucene/analysis/ja/TestFactories.java b/lucene/analysis/kuromoji/src/test/org/apache/lucene/analysis/ja/TestFactories.java
index b05d5edadc3..2fdbfc395d3 100644
--- a/lucene/analysis/kuromoji/src/test/org/apache/lucene/analysis/ja/TestFactories.java
+++ b/lucene/analysis/kuromoji/src/test/org/apache/lucene/analysis/ja/TestFactories.java
@@ -20,9 +20,7 @@ import java.io.IOException;
 import java.io.Reader;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
-import java.util.Arrays;
 import java.util.HashMap;
-import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 import org.apache.lucene.analysis.AbstractAnalysisFactory;
@@ -53,10 +51,8 @@ public class TestFactories extends BaseTokenStreamTestCase {
 
   /** Factories that are excluded from testing it with random data */
   private static final Set<Class<? extends AbstractAnalysisFactory>> EXCLUDE_FACTORIES_RANDOM_DATA =
-      new HashSet<>(
-          Arrays.asList(
-              DelimitedTermFrequencyTokenFilterFactory.class,
-              DelimitedBoostTokenFilterFactory.class));
+      Set.of(
+          DelimitedTermFrequencyTokenFilterFactory.class, DelimitedBoostTokenFilterFactory.class);
 
   public void test() throws IOException {
     for (String tokenizer : TokenizerFactory.availableTokenizers()) {
diff --git a/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/BackwardsCompatibilityTestBase.java b/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/BackwardsCompatibilityTestBase.java
index 04baa47425e..ae5920de368 100644
--- a/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/BackwardsCompatibilityTestBase.java
+++ b/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/BackwardsCompatibilityTestBase.java
@@ -29,6 +29,7 @@ import java.nio.file.Paths;
 import java.nio.file.StandardOpenOption;
 import java.text.ParseException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedHashSet;
@@ -98,7 +99,7 @@ public abstract class BackwardsCompatibilityTestBase extends LuceneTestCase {
       // make sure we never miss a version.
       assertTrue("Version: " + version + " missing", binaryVersions.remove(version));
     }
-    BINARY_SUPPORTED_VERSIONS = binaryVersions;
+    BINARY_SUPPORTED_VERSIONS = Collections.unmodifiableSet(binaryVersions);
   }
 
   /**
diff --git a/lucene/core/src/test/org/apache/lucene/util/TestFilterIterator.java b/lucene/core/src/test/org/apache/lucene/util/TestFilterIterator.java
index a96cf4d40de..275d8e2e9a6 100644
--- a/lucene/core/src/test/org/apache/lucene/util/TestFilterIterator.java
+++ b/lucene/core/src/test/org/apache/lucene/util/TestFilterIterator.java
@@ -17,6 +17,7 @@
 package org.apache.lucene.util;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.NoSuchElementException;
 import java.util.Set;
@@ -25,7 +26,8 @@ import org.apache.lucene.tests.util.LuceneTestCase;
 
 public class TestFilterIterator extends LuceneTestCase {
 
-  private static final Set<String> set = new TreeSet<>(Arrays.asList("a", "b", "c"));
+  private static final Set<String> set =
+      Collections.unmodifiableSet(new TreeSet<>(Arrays.asList("a", "b", "c")));
 
   private static void assertNoMore(Iterator<?> it) {
     assertFalse(it.hasNext());