diff --git a/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php b/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php
index 5eda2bafdfb10..003680a5c074b 100644
--- a/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php
+++ b/src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler/Deprecation.php
@@ -16,7 +16,6 @@
 use PHPUnit\Metadata\Api\Groups;
 use PHPUnit\Util\Test;
 use Symfony\Bridge\PhpUnit\Legacy\SymfonyTestsListenerFor;
-use Symfony\Component\Debug\DebugClassLoader as LegacyDebugClassLoader;
 use Symfony\Component\ErrorHandler\DebugClassLoader;
 
 class_exists(Groups::class);
@@ -81,7 +80,7 @@ public function __construct($message, array $trace, $file)
             }
 
             if ('trigger_error' === $trace[$j]['function'] && !isset($trace[$j]['class'])) {
-                if (\in_array($trace[1 + $j]['class'], [DebugClassLoader::class, LegacyDebugClassLoader::class], true)) {
+                if (DebugClassLoader::class === $trace[1 + $j]['class']) {
                     $class = $trace[1 + $j]['args'][0];
                     $this->triggeringFile = isset($trace[1 + $j]['args'][1]) ? realpath($trace[1 + $j]['args'][1]) : (new \ReflectionClass($class))->getFileName();
                     $this->getOriginalFilesStack();
@@ -323,9 +322,6 @@ private static function getVendors()
             if (class_exists(DebugClassLoader::class, false)) {
                 self::$vendors[] = \dirname((new \ReflectionClass(DebugClassLoader::class))->getFileName());
             }
-            if (class_exists(LegacyDebugClassLoader::class, false)) {
-                self::$vendors[] = \dirname((new \ReflectionClass(LegacyDebugClassLoader::class))->getFileName());
-            }
             foreach (get_declared_classes() as $class) {
                 if ('C' === $class[0] && 0 === strpos($class, 'ComposerAutoloaderInit')) {
                     $r = new \ReflectionClass($class);
diff --git a/src/Symfony/Bridge/PhpUnit/Legacy/SymfonyTestsListenerTrait.php b/src/Symfony/Bridge/PhpUnit/Legacy/SymfonyTestsListenerTrait.php
index c84ec1b66ddbd..7f23b68c2254f 100644
--- a/src/Symfony/Bridge/PhpUnit/Legacy/SymfonyTestsListenerTrait.php
+++ b/src/Symfony/Bridge/PhpUnit/Legacy/SymfonyTestsListenerTrait.php
@@ -23,7 +23,6 @@
 use Symfony\Bridge\PhpUnit\ClockMock;
 use Symfony\Bridge\PhpUnit\DnsMock;
 use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
-use Symfony\Component\Debug\DebugClassLoader as LegacyDebugClassLoader;
 use Symfony\Component\ErrorHandler\DebugClassLoader;
 
 /**
@@ -61,7 +60,7 @@ public function __construct(array $mockedNamespaces = [])
             Blacklist::$blacklistedClassNames[__CLASS__] = 2;
         }
 
-        $enableDebugClassLoader = class_exists(DebugClassLoader::class) || class_exists(LegacyDebugClassLoader::class);
+        $enableDebugClassLoader = class_exists(DebugClassLoader::class);
 
         foreach ($mockedNamespaces as $type => $namespaces) {
             if (!\is_array($namespaces)) {
@@ -82,11 +81,7 @@ public function __construct(array $mockedNamespaces = [])
             }
         }
         if ($enableDebugClassLoader) {
-            if (class_exists(DebugClassLoader::class)) {
-                DebugClassLoader::enable();
-            } else {
-                LegacyDebugClassLoader::enable();
-            }
+            DebugClassLoader::enable();
         }
         if (self::$globallyEnabled) {
             $this->state = -2;
diff --git a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
index 709d6b229155a..48427038b1197 100644
--- a/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
+++ b/src/Symfony/Component/DependencyInjection/Dumper/PhpDumper.php
@@ -12,7 +12,6 @@
 namespace Symfony\Component\DependencyInjection\Dumper;
 
 use Composer\Autoload\ClassLoader;
-use Symfony\Component\Debug\DebugClassLoader as LegacyDebugClassLoader;
 use Symfony\Component\DependencyInjection\Argument\AbstractArgument;
 use Symfony\Component\DependencyInjection\Argument\ArgumentInterface;
 use Symfony\Component\DependencyInjection\Argument\IteratorArgument;
@@ -2193,7 +2192,7 @@ private function getAutoloadFile(): ?string
                 continue;
             }
 
-            if ($autoloader[0] instanceof DebugClassLoader || $autoloader[0] instanceof LegacyDebugClassLoader) {
+            if ($autoloader[0] instanceof DebugClassLoader) {
                 $autoloader = $autoloader[0]->getClassLoader();
             }
 
diff --git a/src/Symfony/Component/HttpKernel/DependencyInjection/AddAnnotatedClassesToCachePass.php b/src/Symfony/Component/HttpKernel/DependencyInjection/AddAnnotatedClassesToCachePass.php
index 5d2703e8cff72..0cbce9b2dff09 100644
--- a/src/Symfony/Component/HttpKernel/DependencyInjection/AddAnnotatedClassesToCachePass.php
+++ b/src/Symfony/Component/HttpKernel/DependencyInjection/AddAnnotatedClassesToCachePass.php
@@ -12,7 +12,6 @@
 namespace Symfony\Component\HttpKernel\DependencyInjection;
 
 use Composer\Autoload\ClassLoader;
-use Symfony\Component\Debug\DebugClassLoader as LegacyDebugClassLoader;
 use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\ErrorHandler\DebugClassLoader;
@@ -93,7 +92,7 @@ private function getClassesInComposerClassMaps(): array
                 continue;
             }
 
-            if ($function[0] instanceof DebugClassLoader || $function[0] instanceof LegacyDebugClassLoader) {
+            if ($function[0] instanceof DebugClassLoader) {
                 $function = $function[0]->getClassLoader();
             }
 
diff --git a/src/Symfony/Component/HttpKernel/EventListener/ErrorListener.php b/src/Symfony/Component/HttpKernel/EventListener/ErrorListener.php
index 8a0d18d8fa4ed..907be45a2b26f 100644
--- a/src/Symfony/Component/HttpKernel/EventListener/ErrorListener.php
+++ b/src/Symfony/Component/HttpKernel/EventListener/ErrorListener.php
@@ -12,7 +12,6 @@
 namespace Symfony\Component\HttpKernel\EventListener;
 
 use Psr\Log\LoggerInterface;
-use Symfony\Component\Debug\Exception\FlattenException as LegacyFlattenException;
 use Symfony\Component\ErrorHandler\Exception\FlattenException;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpFoundation\Request;
@@ -126,7 +125,7 @@ public function onControllerArguments(ControllerArgumentsEvent $event)
         $r = new \ReflectionFunction($event->getController()(...));
         $r = $r->getParameters()[$k] ?? null;
 
-        if ($r && (!($r = $r->getType()) instanceof \ReflectionNamedType || \in_array($r->getName(), [FlattenException::class, LegacyFlattenException::class], true))) {
+        if ($r && (!($r = $r->getType()) instanceof \ReflectionNamedType || FlattenException::class === $r->getName())) {
             $arguments = $event->getArguments();
             $arguments[$k] = FlattenException::createFromThrowable($e);
             $event->setArguments($arguments);
diff --git a/src/Symfony/Component/HttpKernel/Kernel.php b/src/Symfony/Component/HttpKernel/Kernel.php
index 9f85c5bb4e6dc..6732c47fb2d25 100644
--- a/src/Symfony/Component/HttpKernel/Kernel.php
+++ b/src/Symfony/Component/HttpKernel/Kernel.php
@@ -17,7 +17,6 @@
 use Symfony\Component\Config\ConfigCache;
 use Symfony\Component\Config\Loader\DelegatingLoader;
 use Symfony\Component\Config\Loader\LoaderResolver;
-use Symfony\Component\Debug\DebugClassLoader as LegacyDebugClassLoader;
 use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
 use Symfony\Component\DependencyInjection\Compiler\PassConfig;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
@@ -513,7 +512,7 @@ protected function initializeContainer()
 
                 // Remove frames added by DebugClassLoader.
                 for ($i = \count($backtrace) - 2; 0 < $i; --$i) {
-                    if (\in_array($backtrace[$i]['class'] ?? null, [DebugClassLoader::class, LegacyDebugClassLoader::class], true)) {
+                    if (DebugClassLoader::class === ($backtrace[$i]['class'] ?? null)) {
                         $backtrace = [$backtrace[$i + 1]];
                         break;
                     }