About a dozen years ago, one of the forums I now administer was hacked--I was not the admin at that point (although I was on staff), and the forum was hosted at Rackshack, which was unmanaged. The site was recovered from a backup, but one thing that really dragged the recovery into the ground was a single 23,000+ post thread. While rebuilding the search engine in vBulletin, that single thread would be indexing posts at the rate of one or two per second. The admin ended up disabling search on a new forum section, and moved some of our longer threads into it.
Ever since, we have capped threads at 1,000 posts, and after a decade it has become almost part of the forum culture--members expect it, and the staff gets frequent reminders or reported posts to close the old thread and start a new one. These are primarily "throwaway" threads, ones where members post and move on, not reading back. (Such as, "What are you listening to right now?" "What is the weather like at this moment?") Given how efficient XF and ElasticSearch are, I really see no need to have a 1,000 post limit. I may have the staff bump that up to 10,000 (just to reduce their workloads), or not even bother with a limit at all.
I am curious to know if anyone else does this on their forum. And, if there is really a need for it anymore.