This isn't an actual rule per se - it's not mentioned in the Rules board anyway. You do get a warning to consider posting a new thread if a topic hasn't been bumped in 14 days, but the general guideline is more like 30 days. And even that is flexible, depending - like if a major update is posted in a relevant thread or the like. Replying to an old thread to say "Yes I agree" is generally frowned upon. So it's not so much a rule to not necrobump as it is a guideline which depends on context.
As a general rule, past the 30 day mark, it's better to start a new thread on a topic than bump the old one, and useless replies an old threads can result in a lock.