While examining the source code generated by phx.gen.auth, I found a function in user_auth.ex:

  defp renew_session(conn) do
    conn
    |> configure_session(renew: true)
    |> clear_session()
  end

I removed |> configure_session (renew: true) to see how configure_session/2 works, but I didn’t see any change in the behavior of the application.

The official documentation describes the :renew option as follows:

:renew: When true, generates a new session id for the cookie

However, it seems that my browser cookie does not contain a session ID equivalent.
Perhaps this description is out of date and in fact this option has no effect on the cookie.