It is possible to watch specific tags or specific categories, but it is not possible to watch a specific category/tag intersection. For example, there isn’t a way to limit watching the pr-welcome tag for topics in our #feature category while not watching the pr-welcome tag in other categories.
There is a topic from a few days ago that’s related to this issue: Doesn't work: subscribe for topics in the category with a specific tag - #2 by simon.