This seems to have been a request for almost 10 years. Which implies that Zendesk is not going to change their minds about it. Might be nice if you could say more about why. It's not like other systems don't allow annotation of closed tickets.

My use case is probably similar to that of a lot of other customers: we added a Product Area property to allow us to better mine support data for areas that would most benefit from additional development or documentation investment. We took a first stab that those categories, and added an Other for the rest. Now we would like to go back and split out Other into additional categories as well as categorizing the historical tickets that were added before we had Product Area. Having to extract ticket history and then backfill that data in an external system, and report on it in an external system, seems unreasonable.