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When is Thread Necromancy allowed on Forums, or when is it better to start a new Thread?

Generally the only exception to thread necromancy is if it is some sort of informational thread and something new has come up. Like a thread discussing the upcoming release of a game goes dormant for a few months and then some news comes out. That is ok to necro. Same with like if you have a thread in mini painting or something and you post a new mini picture after months of inactivity.

But outside of those two specific cases I would err to just make a new thread.

There's no hard and fast rule. It generally comes down to the individual thread, and whether continuing the previous discussion is going to be more beneficial than starting a new thread, or just going to confuse people due to the earlier participants no longer being around.