Agree, I do this, yes, this is a little frustrating :

    // use Symfony\Component\HttpFoundation\Session\Session;
    /** @var Session $session */
    $session = $request->getSession();

wait.. nothing told that $this->get('session') returns an instance of Session. So, replacing $this->get('session'); // returns mixed by $request->getSession() //returns SessionInterface is absolutely identical. In both case you can't be sure that the returned object will be an instance of Session but in both case you can be confident enough, that in the context of your application you'll get an instance of Session.

This issue is not related to Session deprecation (has the behavior remains the same). And is a duplicate of: #39222, #20258, #11279, #10036, #5568, #39222

But the deprecation will remove also session.flash_bag from the DI. So it's related for me :D .

Thumbs up! I'm also getting an error on Psalm for this:

ERROR: UndefinedInterfaceMethod - src/Helper/FlashMessageHelper.php:34:25 - Method Symfony\Component\HttpFoundation\Session\SessionInterface::getFlashBag does not exist (see https://psalm.dev/181)
        $this->session->getFlashBag()->add('success', $message);

What about creating a WithFlashBag interface that holds public function getFlashBag(); that can used as an intersection type on PHP 8.1: SessionInterface&WithFlashBag