Your question contains it’s own answer, in that if it is against the CoC, such as trolling or harrassment, the it may be actioned upon.
Does the post itself contain information that might be as relevant now as it was when originally posted.
Does it help, or simply say “Me too”.
Is it relevant to the forum it’s in.
Suffice it to say, if a post is over 2 weeks old it might be time to open a new thread.
Usually the thread is locked or deleted if a mod finds it does not fit to the game currently. I’ve seen posts commented on by support agents that were brought back from the deep with no issues. But if you find it is old, create a new thread is fine. Even referencing the original is also fine.