Not sure if COTH has any leeway to alter forum function, but what if you write a validation query that runs in the background when you click Reply, where IF (today) is >1 year (or whatever) from the date of the OP, THEN user gets a simple dialog box that they have to click through. “This thread is more than a year old. Do you want to continue?”
This would help limit the inadvertent resurrection of Zombie Threads.