Since the introduction of the "simple security model", a user with "some basic forum rights" should be able to create a thread (or forum, on EE with multi-site or multi-locale).

What means "some basic forum right"?
The issue poped-up because currently anonmyous user cannot create a 1st comment on page-commenting.
When entering the 1st comment to a page, a thread-node is created in the pagecommenting-Forum-Node.
On EE, with multi-site and/or multi-locale, it is required to create a forum-node by anonymous user.

Note, that anonymous-user has the role "forum-pagecomments-user".
Since we do no more check speficially for every created forum (per ACL), it should be possible to create a thread with the role "forum_ALL-user".
Now ... either anonymous-user also should get "forum_ALL-user" (which i think is wrong) ... or for the creation of a thread, it should be checked, whether the user has
"forum-pagecomments-user" or "forum_ALL-user" -role.

During the very last changes on MGNLFORUM-253 there has been added a check in DefaultForumManager#createThread.
isModerator currently prevents anonymous-user to create the thread (or the forum-node).