I can create a PR for this, but I'm not sure if there was a reason it was left off the interface?

I searched through the issue history for the method name and could not find any reference to the said method's absence.

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/SessionInterface.php#L207 and https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/Session.php#L248

And the PR would go to master, correct?