Thank you for this PR.
I'm afraid it is a BC break to add a method to an interface. You cannot target 2.7 branch for this PR.

Also, this is not a bugfix, it is a new feature.

Duplicates #22395 and many other

You can type hint Session instead of the interface - I doubt there is real competition for other implementation of SessionInterface, isn't it?

I think the main reason for typehinting the SessionInterface is the ability to mock or use an adapter for legacy applications.

What if we introduce an interface like SessionWithFlashBagInterface that extends SessionInterface? This would make type hinting possible for and not forcing FlashBag support upon every implementation.

Closing as this has been discussed a lot already and cannot be done for BC reasons. Note that even without BC issues, this addition was rejected back then.