When we start using the "formatter URL" statements on properties to generate HTML links to authority vocabularies, we need a way to purge such cached renderings when the respective statement in the property definition changes.
As an interim workaround, I've put together a quick-and-dirty script to slowly do a rolling purge of items using pywikibot. It identifies all items that have not been edited since before the formatter URL was changed, generates a list, and works through them. As it uses the PWB framework it respects maxlag, and will back off if overloaded.
Works fine for small sets of items, but maxlag-related delays mean it's probably not a practical solution for very heavily used properties.
It has occurred to me today that this problem is (in some ways) a blessing in disguise - it helps mitigate against the effects of vandalism by deleting or changing a formatter URL.
If we do manage to solve this eg by having a server-side script purge items after the formatter URL changes, maybe it would be good to build in a natural delay - say it only runs twelve hours after the changes, or checks for new formatter URLs once a day, or something.
After purging a property and an item where the property is used, it still seems to be cached somewhere, and if a property is added after the formatter URL is changed, the link created is based on the old formatter URL.