I'd prefer to add a new column to the user table, in which the user-status (deleted|banned|etc.) could be saved. even if the user-information is finally deleted from the db, what can be said against marking postings with "user deleted"?

I'll think about it. But so far I consider the user data table as an addition. All for the conversation important data should IMHO be in the entry table.

ok, for me one of the the advantages of your script was it's flexibility...
what are your reasons for this change?

If you have enabled both thread and board view, the users of the board view often "destroy" the threads since they tend to reply predominantly to the message stated below. Furthermore I think if there's a possibility to open the whole thread there's no need for the board view. Above all if you don't want to see all the threads on the index page you can now fold them.