I just noticed that I misunderstood your question and gave you the wrong answer.
There is no ACL in v6 yet, so you have to comtrol it with Joomla ACL. You need to set the access level of your menu to match the acces level of your allowed users. If you set the menu item to Public everybody can see the newsletters.
And you're right, the module to display newsletters is not available for v6 yet. Acyba will bring it back for sure, we just need some patience.