diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index e34082d60ce9..c2333519fa15 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -132,6 +132,7 @@ Other
 * GITHUB#14817: Refactor some complex uses of PriorityQueue to use Comparators. (Simon Cooper)
 
 * GITHUB#14607: Index open performs version check on each segment, ignores indexCreatedVersionMajor (Rahul Goswami)
+* GITHUB#15454: Restore binary readability for 8.x indexes like previously and throw IndexFormatTooOldException for an older default codec not found on classpath (Rahul Goswami)
 
 ======================= Lucene 10.4.0 =======================
 
diff --git a/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestAncientIndicesCompatibility.java b/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestAncientIndicesCompatibility.java
index af3cf28c93ad..246e6fe01b27 100644
--- a/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestAncientIndicesCompatibility.java
+++ b/lucene/backward-codecs/src/test/org/apache/lucene/backward_index/TestAncientIndicesCompatibility.java
@@ -45,7 +45,6 @@
 import org.apache.lucene.tests.util.LuceneTestCase;
 import org.apache.lucene.tests.util.TestUtil;
 import org.apache.lucene.util.IOUtils;
-import org.apache.lucene.util.Version;
 
 public class TestAncientIndicesCompatibility extends LuceneTestCase {
   static final Set<String> UNSUPPORTED_INDEXES;
@@ -200,7 +199,7 @@ public void testUnsupportedOldIndexes() throws Exception {
       checker.setInfoStream(new PrintStream(bos, false, UTF_8));
       checker.setLevel(CheckIndex.Level.MIN_LEVEL_FOR_INTEGRITY_CHECKS);
       CheckIndex.Status indexStatus = checker.checkIndex();
-      if (getVersion(version).onOrAfter(Version.fromBits(8, 6, 0))) {
+      if (version.startsWith("8.") || version.startsWith("9.")) {
         assertTrue(indexStatus.clean);
       } else {
         assertFalse(indexStatus.clean);
@@ -218,18 +217,6 @@ public void testUnsupportedOldIndexes() throws Exception {
     }
   }
 
-  private Version getVersion(String version) {
-    if (version.startsWith("5x")) {
-      // couple of indices in unsupported_indices.txt start with "5x'
-      return Version.fromBits(5, 0, 0);
-    }
-    String[] versionBitsStr = version.split("[.\\-]");
-    return Version.fromBits(
-        Integer.parseInt(versionBitsStr[0]),
-        Integer.parseInt(versionBitsStr[1]),
-        Integer.parseInt(versionBitsStr[2]));
-  }
-
   // #12895: test on a carefully crafted 9.8.0 index (from a small contiguous subset
   // of wikibigall unique terms) that shows the read-time exception of
   // IntersectTermsEnum (used by WildcardQuery)
diff --git a/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java b/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java
index c6042baf0b03..dac837a30801 100644
--- a/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java
+++ b/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java
@@ -328,7 +328,7 @@ public static final SegmentInfos readCommit(
         throw new IndexFormatTooOldException(
             input, magic, CodecUtil.CODEC_MAGIC, CodecUtil.CODEC_MAGIC);
       }
-      format = CodecUtil.checkHeaderNoMagic(input, "segments", VERSION_86, VERSION_CURRENT);
+      format = CodecUtil.checkHeaderNoMagic(input, "segments", VERSION_74, VERSION_CURRENT);
       byte[] id = new byte[StringHelper.ID_LENGTH];
       input.readBytes(id, 0, id.length);
       CodecUtil.checkIndexHeaderSuffix(input, Long.toString(generation, Character.MAX_RADIX));
@@ -529,11 +529,13 @@ private static Codec readCodec(DataInput input) throws IOException {
     } catch (IllegalArgumentException e) {
       // maybe it's an old default codec that moved
       if (name.startsWith("Lucene")) {
-        throw new IllegalArgumentException(
+        throw new IndexFormatTooOldException(
+            input,
             "Could not load codec '"
                 + name
-                + "'. Did you forget to add lucene-backward-codecs.jar?",
-            e);
+                + "'. "
+                + e.getMessage()
+                + ". Did you forget to add lucene-backward-codecs.jar?");
       }
       throw e;
     }