Sometimes it’s useful to maintain a read-only category accessible to one or more groups to follow announcements or mirror a diffusion list.
Problem
Since admins can post anywhere, it can happen that an admin will reply or post a topic in such a read-only category unknowingly, resulting in lost discussion or unnoticed intervention, or even in unsolicited email back to the original mailing-list.
Resolution
When an admin tries to reply to an existing topic, or tries to create a new topic in a read-only category, they should receive a big fat warning greying out the composer and asking them to confirm that they indeed want to do this, suggesting they could quote the topic in another, more appropriate space.
There is this TC for the closed topics. Might be able to extend it or create an additional custom Theme component for the create topic in s closed category.
Maybe though to fit more closely might need to see if can change a bit to check a category for see only permission in category security.