So, a user should go read the whole old thread, 678 posts, to see of his/her post is relevant in adding something new?
Warnings that can be used preventing gravedigging:
‘You really want to gravedig?’ 'Your username will be changed permanently to ‘necromancer’.!
Necromosan
We cant expect people to do their reading, half the flob would fall off
The key is adding something on-topic and substantial. In other words, there are good gravedigs and bad gravedigs. If you’re not familiar with the tread content, there’s always some risk of being off topic.
There goes the other half! 
Among the latter: reviving a long-dormant thread to ask a question to (or give advice to) a poster who is loooooong gone from this site. 
Guy
Tread lightly 
Some people just stopped posting, but are still lurking?
Well… they must be lurking without an account cause if it says they haven’t signed in since '06…I think it’s safe to say they moved on.
Well, I don’t think gravediggers will check the last time people signed in, or do they?
I would.
If I am replying to something someone posted 20 years ago.
20 years!?! Well, in that case, I would probably too.
Diggers got to dig.
Yes, I even did a grave dig here to post that. 
Plus I have a cake next to my name today, so it is extra special.
Guy
Did you get a shiny new spade for your cake day?
Wait not that kind of spade! 
Guy