Something may be not right here, but I am quite against the pattern of showing “New Topic” greyed out. It is confusing and leads to questions long term.
My call here would be just to show the new topic button unconditionally, rather that try being fancy about greying it.
Then on click we could display a modal
Sorry you do not have access to posting topics in the #feature:announcements category.
@codinghorror what are your thoughts here? Should we change it so we show the button and let logged on accounts click and learn why they can not post topics?
Regardless the tagging control should not be messing with the permission check, but stuff gets ultra complex with tag/category pairs. You may have permission to post topic in a category but no permission to use the tag.