@warslett I've found a single occurrence of these deprecated services in the Symfony Docs. It's this example about "namespaced session attributes":

.. code-block:: yaml

    # config/services.yaml
    session:
        public: true
        class: Symfony\Component\HttpFoundation\Session\Session
        arguments: ['@session.storage', '@session.namespacedattributebag', '@session.flash_bag']

    session.namespacedattributebag:
        class: Symfony\Component\HttpFoundation\Session\Attribute\NamespacedAttributeBag

How could we remove the '@session.flash_bag' service in this config? Thanks.

@javiereguiluz I will test this to be sure but in theory you don't need to inject a flashbag. If you want to use the default flashbag, you just set the flashbag parameter to null. The deprecated flashbag service uses session as a factory so it will always be the flashbag contained in the session. I'd suggest this as the recommended approach for injecting NamespacedAttributeBag:

# config/services.yaml
    session:
        public: true
        class: Symfony\Component\HttpFoundation\Session\Session
        arguments: ['@session.storage', '@session.namespacedattributebag', ~]

    session.namespacedattributebag:
        class: Symfony\Component\HttpFoundation\Session\Attribute\NamespacedAttributeBag

EDIT: Updated muddled wording

or even just:

# config/services.yaml
    session:
        public: true
        class: Symfony\Component\HttpFoundation\Session\Session
        arguments: ['@session.storage', '@session.namespacedattributebag']

    session.namespacedattributebag:
        class: Symfony\Component\HttpFoundation\Session\Attribute\NamespacedAttributeBag

@warslett thanks a lot for double checking this! We've created a pull request to fix this. Thanks!