en.wikipedia.org/wiki/Eternal_September
It seems some new members do not understand that threadomancers are frowned upon, if only for the smell of undeath that permeate their followups. If you aren't knowledgeable in netiquette, maybe you should take hints from the local community.
If you want to refer to something old, start a new thread and just add https://forum.atlas-games.com/t/hoh-mc-faerie-glamour-question/3875/1 to make it clear where you are coming from.
While I realize that putting s new reply on to the end of a long thread can sometimes be an impediment to fresh discussion as the words of the past dominate the discussion of the present and people new to the issue might slog though a few pages of old discussion to get to new material, I personally would very frequently prefer the continuation of an old topic to be continued on the same thread. Oftentimes there's much of value that has been said by previous posters and links are frequently ignored leading us to retread the same ground again and again.
(Carefully reading and analyzing previous threads and then crafting a clear, instructive and 100% accurate summation of the state of the issue every new thread would of course be the best of all possible options, but I personally haven't had the Creo Mentem rituals to raise my communication to +5 performed and I don't live in a time dilation regio.)
Netiquette for the rest of the world can be damned, in my opinion posters should use their best judgement and do what is best for discussion. This might well involve thread necromancy.
Please note that this is just my personal opinion. Pay no mind to my spaminator status it doesn't give me any special authority regarding appropriate message board behavior.
Thank you for being more eloquent than I, Erik. Oh, and thanks for agreeing with me 
. I was refraining from being the first to respond, since I found the tone of the OP to be a little condescending and elitist. I was certainly unconcerned about others having to "slog through a few pages of old discussion" in the instigating example of thread necromancy, as it was less than page's worth of posts.
I was going to declare my agreement with Erik, but instead I will simply copy this post, for it says it all.
Starting new threads for the same topic every 2 months just means I have to collect and paste-in links to that many more threads when something pops up again.
I don't mind thread necromancy either, as long as the person performing it is aware that he's trying to revive a bygone discussion by using an existing thread for his needs, and doesn't attempt to answer a question or solve a problem that another person - that might not even read the forums anymore - postulated ages ago.
So to sum up:
I feel I have to break a lance in favour of the original poster, Tugdual. Resurrecting a long-dead thread is generally considered bad form on any forum (in fact, the Ars Magica forum is pretty tolerant in this regard).
And like with all aspects of netiquette, there's a reason: "threadomancy" causes other participants to waste time and effort, which is annoying. This happens in several ways. The main one is that resurrected threads force participants to reread a lot of stuff to make sure they have the right context when responding (the alternative, responding to the last post without looking at what's been said before, is bad form for its own reasons). The second is that things may have changed, sometimes radically, from when the thread started; so one not only needs to read the whole stuff, but also mentally "fast forward" it, which takes effort (this is particularly true for a rapidly changing field like an rpg game line). Note that this second issue is still true even if the resurrected thread was very short. These are the two main issues, I'd say, but there are a lot of other minor ones (e.g. resurrected threads briefly mislead you about how "hot" a topic is: you suddenly see a 135 posts thread pop up that wasn't on the first page 24 hours ago, and you think "oh wow, this is definitely worth a lot of my attention!").
And there is virtually no advantage in resurrecting an old thread compared to starting a new one with the relevant links and a brief summary of what they say (yes, starting a new thread without referring what was said before is also bad netiquette 
). Erik Tyrell says that people won't follow the links, but that totally misses the point, in my opinion. The point is that a new thread, with links and a very short summary of previous discussion, is the most convenient form for other participants to access the information: they may choose to ignore everything, to just read the summary, to skim through the linked discussions - whatever suits them. And ultimately, netiquette is about being considerate towards other people, and not doing stuff that might irk them, even when some or most of them wouldn't really mind.
What ezzelino said.
+1
On the other side, I wouldn't throw someone from the Order for threadomancy. I'll just be slightly annoyed that he was inconsiderate.
Also +1
Threadomancy is just a small breach of the Peripheral Code. Depending on the Tribunal's vote, the perpetrator may be condemned or not. And if he's condemned, the offender may be let off with a simple warning, or be fined a single pawn of vis. 
Well, a single undead minion should be allowed per necromancer. I don't care if they carry proxy votes from the Rhine Tribunal.
Seriously, everyone does mistakes from time to time. The trick is not to do them twice.
Not that old fogeys ever derail threads, repeatedly.
This is a remarkable thread... because you guys are different than others!
I know a german roleplaying-games-forum with members, that are thinking just the other way. Their point is: If you don´t use older threads to continue a discussion, you have too many superfluous posts, because the same thoughts will be posted over and over again. Furthermore no one is able to locate posts about certain topics. The main difference is, that these guys understand their forum a little bit like a work of reference, something that is unthinkable concerning the Atlas Games Forum.
There are moments, I feel sorry for the many ideas in the Atlas Games Forum that will be swept under the carpet soon after their publication.
On the other hand your Forum structure is a sign for a more lively game with a certain amount of fluctuation... this is an aspect I appreciate.
Chiarina.
This is how I prefer to think of things as well.
On this particular matter, I must disagree with Tugdal most strongly.
The no-threadomancy thing works though, because this then feels like a debate between magi. After all, no-one expects the new apprentice to have ANY idea what his/her master/mistress is on about in their debate on corpus that's been ongoing for 10 years
But I'm not sure if the cut-off has to be so strict. Maybe about 18 months would be reasonable? 
Bob
Did you mistype 19?
Well, I just realized that I revived several old threads. Hope it did not annoy anybody, especially as some of my posts were not overly interesting... So, I'll take care.
It's not a big deal. You're new. Welcome to the forum.
Practicing threadomancy on a thread that complains about threadomancy ... it has a perverse allure that I could not resist
@jonathan: no the 18 was deliberately too short
I had been planning to ressurect this thread ever since it petered out, terminus est's responses today just made it the perfect opportunity
@ terminus est: Indeed welcome to the forum! we are quite playful and nice her, so please feel welcome, I'm sorry if I caused offence.
Bob
I like thread necromancy.
But I would, wouldn't I.