In the context of the text I have some remarks:

In the template of the next page (or even better, in your base layout template), read any flash messages from the session:
- controller.rst

For example, one common problem in this situation involves checking for flash messages, which are stored in the session. The following code would guarantee that a session is always started:
- avoid_session_start.rst

Minor details, but both of them are still referring to using the session, while the examples do not use the session directly anymore (no reference to app.session). Maybe those 2 pages could do with a note that this is a (new) shortcut?

Besides @iltar's comment, we should also add a versionadded directive after the code example:

.. versionadded:: 3.3
    The `app.flashes()` method was introduced in Symfony 3.3. Prior to version 3,3
    you had to use `app.app.session.flashBag`.

Status: needs work

We should also deprecate or refactorize or add a big warning message to this article: https://symfony.com/doc/current/session/avoid_session_start.html

@javiereguiluz I think we should deprecate it because your new method does not longer checks the session making that part of the documentation confusing and not longer needed.

hasPreviousSession() still can be use-full imo. So I would keep the article by adding a small .. tip:: saying that there is this new nice flashes() method that can be used instead. We can always update the example if we can come up with a better one in the future.

Regarding the text, I think we can leave the session bit in there and do something like: "In the template of the next page (or even better, in your base layout template), read any flash messages from the session using app.flashes():" Not 100% sure though

By the thumbs up from @iltar I assumed the suggestion from @wouterj seems to be correct.

Regarding your hasPreviousSession() it is still useful but that makes these lines (^{source file}) not needed imo. Should I replace them with the small .. tip:: Since Symfony 3.3 there is a better method to read the flash messages from the session. See controller.rst#flashmessages... something like that?

Thank you @Ricknox.