I looked on the French help forum and it says:
"Si un sujet dépasse 1100 messages, une fonction non paramétrable divisera automatiquement le sujet : il sera séparé en un sujet de 990 messages, (qui sera verrouillé) et un sujet de 110 messages portant le même titre."
If a topic exceeds 1100 posts, an unsettable function will automatically divide the topic: it will be divided in a 990 posts topic, (which will be locked) and a 110 posts topic with the same title.
You could add the link to the old topic in the first post of the new one and add a number in the title of the new topic like I've seen people do on the old forum. You can also create a read-only archive thingy where old topics can be either manually moved or automatically moved after a set amount of time without answers, it was named the "Corbeille" (trash bin) when I used to admin a not very active Forumactif/motion.