During our DTM --> Launch conversion we discovered a handful of rules that no longer applied due to code changes or our content authors stopped using a component in favor of another.
I'm not sure how I'd do this ... But I'd like to entertain the idea of tracking what rule names actually get fired (over time). Then in the future, we could track the rules reported as run (and quantity) and correlate that back to the rules in Launch. Any rule that was fired zero times then gets investigated as a defect, or an alert to our analytics/product managers as a candidate for functionality to possibly get rid of. (Or conversely - promote)
We normally catch these defects during our product lifecycle, but we do have extended release timeframes, or releases which require months of waiting for content editors to catch up. (Which makes these defects hard to catch)