what about 2.15.x branch?

That one is for bug fixes only

php 8.4
2.16.x branch
config:

    orm:
        auto_generate_proxy_classes: true
        enable_lazy_ghost_objects: true
        report_fields_where_declared: true
        validate_xml_mapping: true
        naming_strategy: doctrine.orm.naming_strategy.underscore_number_aware
        identity_generation_preferences:
            Doctrine\DBAL\Platforms\PostgreSQLPlatform: identity
        auto_mapping: true
        enable_native_lazy_objects: true
        mappings:
            App:
                type: attribute
                is_bundle: false
                dir: '%kernel.project_dir%/src/Entity'
                prefix: 'App\Entity'
                alias: App
        controller_resolver:
            auto_mapping: false

composer update --with-all-dependencies

Executing script cache:clear [KO]
 [KO]
Script cache:clear returned with error code 1
!!
!!  In ProxyCacheWarmer.php line 49:
!!                                                       
!!    Unable to create the Doctrine Proxy directory "".  
!!                                                       
!!
!!
Script @auto-scripts was called via post-update-cmd

Hm maybe we should unset these settings only if enable_lazy_ghost_objects = false?

I'm heading out of town so can't help more right now, but lazy ghosts probably shouldn't be enabled alongside native ghosts. But that's also a little weird because the settings in this PR and that lazy ghost option are in the "don't configure this for each EM" list, but native ghosts can be configured on each EM separately.

The cache warmer issue looks to be a bug in symfony/doctrine-bridge, this block always tries to create the proxy directory and doesn't handle a null return from the object manager configuration's getProxyDir() method (both the ORM and MongoDB ODM have nullable returns, the PHPCR ODM defaults to the system temp folder so it won't return null).

Fixing some of this would be easier if the enable_native_lazy_objects option were in the list of keys that isn't configured on each entity manager separately, but that ship has sailed (because that'd also give a way to approach not enabling the deprecated proxy autoloader as well). So right now, if a Symfony application running ORM 3.4 or later on PHP 8.4 has multiple entity managers, and at least one of them doesn't have the native lazy objects option enabled, the cache warmer and proxy autoloader need to be active.

is it too late to chang that option to make it global? it would be a bc-break but i don't think many people have used it yet...As you say it would make it easier to also fix the other deprecation notice