diff --git a/RELEASE-NOTES-1.42 b/RELEASE-NOTES-1.42
index b623f9cd6ae..02acbcb2d42 100644
--- a/RELEASE-NOTES-1.42
+++ b/RELEASE-NOTES-1.42
@@ -621,6 +621,8 @@ because of Phabricator reports.
   Vuex 4 will remain accessible for the foreseeable future. Pinia should be used
   for new projects.
 * Linker::makeHeadline() has been deprecated.
+* SerializedValueContainer::isUnified() has been deprecated and now emits
+  deprecation warnings since 1.42.
 * Linker::generateTOC(), Linker::tocIndent(), Linker::tocUnindent(),
   Linker::tocLine(), Linker::tocLineEnd(), Linker::tocList() are deprecated.
 * Title::getBrokenLinksFrom() has been deprecated.
diff --git a/includes/libs/objectcache/MediumSpecificBagOStuff.php b/includes/libs/objectcache/MediumSpecificBagOStuff.php
index d6ba87b332e..c169d3e2e1f 100644
--- a/includes/libs/objectcache/MediumSpecificBagOStuff.php
+++ b/includes/libs/objectcache/MediumSpecificBagOStuff.php
@@ -801,10 +801,6 @@ abstract class MediumSpecificBagOStuff extends BagOStuff {
 	 * @return string|null|bool The combined string, false if missing, null on error
 	 */
 	final protected function resolveSegments( $key, $mainValue ) {
-		if ( SerializedValueContainer::isUnified( $mainValue ) ) {
-			return $this->unserialize( $mainValue->{SerializedValueContainer::UNIFIED_DATA} );
-		}
-
 		if ( SerializedValueContainer::isSegmented( $mainValue ) ) {
 			$orderedKeys = array_map(
 				function ( $segmentHash ) use ( $key ) {
diff --git a/includes/libs/objectcache/serialized/SerializedValueContainer.php b/includes/libs/objectcache/serialized/SerializedValueContainer.php
index 3203ef5abd1..4a6754bd2e9 100644
--- a/includes/libs/objectcache/serialized/SerializedValueContainer.php
+++ b/includes/libs/objectcache/serialized/SerializedValueContainer.php
@@ -12,6 +12,9 @@ class SerializedValueContainer {
 	// 64 bit UID
 	private const SCHEMA_SEGMENTED = 'CAYCDAgCDw4';
 
+	/**
+	 * @deprecated since 1.42
+	 */
 	public const UNIFIED_DATA = '__data__';
 	public const SEGMENTED_HASHES = '__hashes__';
 
@@ -27,10 +30,12 @@ class SerializedValueContainer {
 	}
 
 	/**
+	 * @deprecated since 1.42.
 	 * @param mixed $value
 	 * @return bool
 	 */
 	public static function isUnified( $value ) {
+		wfDeprecated( __METHOD__, '1.42' );
 		return (
 			$value instanceof stdClass &&
 			( $value->{self::SCHEMA} ?? null ) === self::SCHEMA_UNIFIED