I started looking at this issue last night in the context of a larger project I’m working on.
First, after looking at the relevant code for this I would reiterate what the Discourse team has already said. Implementing this specification would be a huge hairy change. From a ‘core’ perspective I think It makes complete sense to postpone it, and maybe not do it at all.
For me at least, it seems that the issue is really one of triage. I need the ability to limit a moderator’s attention to a particular category. I want them to be able to focus on that category without getting distracted by notifications from other categories.
Completely restricting access to moderation actions outside of a assigned category seems like more of an edge case. If someone is going to get moderation powers, and be told that they are assigned to a specific category, it is unlikely that:
a) they will regularly exercise their powers outside of their assigned category; and
b) if they do, it is unlikely that this exercise will conflict with the overall moderation goals of the site.
In the unlikely event of both ‘a’ and ‘b’ being incorrect, then they probably shouldn’t be a moderator anyway.
Indeed, there may well be situations in which moderators assigned to a particular category will provide useful support in categories in which they are not assigned to.
So I’ve started a plugin I call “Category Moderator Lite” to implement this narrower ‘workflow-orientated’ version of category-specific moderation.