This is not marking the parameters as deprecated to warn places using them.

You mean that the !isset($config['default_uri']) should be removed from the condition? Or is there a better way?

I mean actually deprecating the parameters: https://symfony.com/blog/new-in-symfony-6-3-dependency-injection-improvements#deprecating-container-parameters

Thanks for the hint. I have updated the patch.

I wouldn't deprecate router.request_context.base_url (the others I would).
Instead, I'd fix it this way:

--- a/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
+++ b/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
@@ -1334,8 +1334,7 @@ class FrameworkExtension extends Extension
         $container->setParameter('request_listener.https_port', $config['https_port']);
 
         if (null !== $config['default_uri']) {
-            $container->getDefinition('router.request_context')
-                ->replaceArgument(0, $config['default_uri']);
+            $container->setParameter('router.request_context.base_url', $config['default_uri']);
         }
     }

Wouldn't this be a potential source of confusion? If a user sets router.request_context.base_url to "example.com", the default_url (maybe via an ENV variable) to something else and at the end, base_url will have another value than what he defined?

And what would be the advantage?

The advantage is not breaking things (deprecating first but still) and allowing bundles to read the value also.
The confusion would be the responsibility of the app, not ours.

Historically the router.request_context.base_url was used to specify only the URL path (for example here or in the older version of the symfony doc).

Imho overwriting the base_url with the default_uri, which is always an absolute URI containing scheme and host, could lead to a hidden BC break.

This was later improved to support full URI, see #36651
This also matches the fact that the config value is injected at the same position as the param, which means that today already, one can set router.request_context.base_url and basically configure the default_uri

Okay, I have applied your suggestion ;-)