When a topic gets auto-closed the system posts this text: This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.
May I suggest to change the text slightly to: This topic was automatically closed 180 days after the last reply. If you have something substantial to add, you can use the "Flag" button, to ask a moderator to re-open the topic
or something like that, to make it known that exceptions are possible to continue adding information to a certain topic. I was unaware of this possibility for years.
I experienced first hand how fast the moderators are when asking to re-open a topic. And I got some advice for free that this topic was open for suggestions.
Anything with the same effect as the text suggested in the OP would be a big improvement.
However, I don't think you can flag an entire thread, only an individual post, so maybe suggesting that the last post should be flagged would be the easiest.
Whilst you cannot flag and entire topic you can explain why you want the topic reopened in the text box associated with the "Something else" dialogue associated with the flag
OK. But to get to that dialogue, you have to select a post to flag. Of course it could be any post in the thread but, for the purpose of formulating a suitable replacement "closed" message, I suggest flagging the most recent post.
I suggest that we do not get bogged down in the details. In many cases it would make sense to flag a post other than the last one in a topic, but the suggestion to flag the topic to be reopened needs to be short, easy to understand and easy to execute.
Suggesting that the last post in a topic is flagged would meet this criteria, particularly as you have to be at the end of the topic in order to see the message
