So this is something I’ve started trying out and I think it’s working fairly well. It’s not terribly easy to do, but I think there are times when it’s worth doing — particularly when it’s not possible to perform a sensible topic split without it. I wanted to explicitly note how I did it, both for future-Matt and other moderators/admins who want to do it, and also to officially remark that this is a thing that we do now.
Here’s what I’ve been doing:
- Copy the portion of the post to split out. I actually hit edit* and copy the raw text.
- Create new topic in a private staging area with the contents of the post
- Transfer ownership of the new post to the OP
- Change the date of the entire topic to the date of the original post. Note that Discourse doesn’t allow changing the timestamp of a single post, but it’ll happily shift all the posts in an entire topic by some delta such that the first post lands at the given timestamp. So it’s important to do this when there’s still only one post in the topic.
- Select all the subsequent posts and merge them into the dedicated topic
- Move the new topic out of the staging area into the appropriate category
- Edit the original post and add
<details><summary><em>Portion manually split into <a href=/t/topicid>new topic title</a></em></summary>(you can’t mix markdown in those HTML tags) - Add a staff notice to the new topic that it was started from a topic split, linking back to the original post.
Both the original post and the new post get 
edit marks noting the addition of the <details> block and the change in ownership. The person whose post was manually split in this manner will get two notifications — that the new topic was moved and that their original post was edited:
As you can see it’s quite involved. And the timestamp muckery makes it highly impractical to manually split multiple posts from the same topic.
So, all the more reason to keep your posts on topic! If a particular conversation thread starts bifurcating, that’s ok! You can even respond to the multiple topics, but you do us moderators a huge favor if you just split your post ahead of time, posting twice.