Disable Topic Subscription Pruning for Pinned Topics

From the General Settings, "Auto prune all topic subscriptions if the topic has no replies over [x] days". This is well and good, especially for those who are limited in database storage or performance, but it doesn't make sense to prune subscriptions to pinned topics. For example, my subscription to the pinned topic in this forum on Skin Interface Suggestions had been pruned. A new option (default hopefully) to disable this for pinned topics would be nice.