Was the old topic an imported one? We made the decision to close those topics for a few reasons.
While the import did a good job of creating threads - it had no way to understand when people went off topic - it uses the message id’s to build threads, and on the mailing list it was quite common for a single thread to actually have a bunch of mini threads - people were not disciplined in creating new threads but just replied to existing ones (I’m sure we all did that at some point).
Also while we imported users who posted to the threads, not everyone is still with us or still active. Since imported users were set to mailing list most, allowing replies ran the risk of people suddenly getting email notifications out of the blue.
There were probably other reasons which I forget.
Anyway, the upshot is, create a new topic, in the appropiate category - you can always posts a link to the original topic (bearing in mind only adug members have access to the imported topics) and you can also tag members using @Lachlan etc to draw their attention to the post if you feel it’s important they see it.
As far as I can tell, we do not have auto close enabled for non imported topics.
One reason for auto closing topics is to reduce spammer activity. A common spammer technique is to reply to an old topic, with a reply that looks legit (probably copied from somewhere else by googling the topic) but has spammy links. I get these on my forums all the time (we don’t auto close either) and I’ve been fooled by them a few times.