DJ, this is why:
https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=11778
In this thread, a certain person asks a question, the other awnsers, and nothing for something like 8 months, then the person which made the thread necro-bumped it just to thank him.
This is ridiculous, and could be stopped with this idea implemented into the forums.
I understand why you want it added in, I'm just saying that 2 months still allows for a lot of necro. And anyway, some threads, even though they may not have been commented on for a while, still need answering or require extra information.
True story.. and turn this into moderator job would be too much work for moderators. Maybe we should have specific people in forums to do this?
Or just do what has always been done, tell people not to necro, and if they do without good reason ban them for a bit :3