not to post to all members

[RGSMDNR]

how about something where if a mod or the admin posts in the thread, they have the option for that message not to be displayed to all members...

[Oldiesmann]

Making topic-level permissions like that would make things more complicated. SMF just does it on a board-by-board basis. If you want to post threads that only certain users can see, then only give those certain users the option to view the board that thread is in, or send it via PM.