I believe this was moved to the centralized permissions page. Select your category from the drop-down and configure which groups can access it.
Posting threads and topics only visible by specific group
Does anyone know if this is possible with the stock tools or if there is currently a plugin to acheive this?
The ability to select visibility of certain posts, for example an announcement, visible only to those in a specific group. Or a way to create categories only available to members of a specific group.
You can already create a category only visible to a certain group by creating such a group and selecting the privileges in the ACP.