most issues in the HttpFoundation Component are related to Sessions.

The HttpFoundation is described as:
The HttpFoundation component defines an object-oriented layer for the HTTP specification.

if you read the http specification there is nothing related to sessions, even more sessions are violating the idea of a stateless protocol.

Sessions are just a concept how to use cookies in combination with a server side key value store.
If you take a look at the HttpFoundation, now it knows a lot about our server side key value stores like the filesystem memcached, pdo, mongo, memcache. this has nothing todo with "an object-oriented layer for the HTTP specification".

i don't want to say that we should not support the concept of sessions. the idea is just to accept that sessions are not related to http.

consider the difference:

 $request->getSession()->get('foo')

vs

$sessionStorage->byRequest($request)->get('foo')

so it would be up to any kind of session storage to know about rfc6265