Not sure if there might be a lof of need for such a feature.
Quite often, in our forum people reply to a previous message in a topic and raise a new topic in the next paragraph of the same reply.
Actually, that’s us who teach users not to write multiple replies but rather use “quote reply” and put everything in a single reply.
Now, if I want to keep one piece of the message in its topic and move another piece of the message (and some of the following replies) in a separate topic, how do I do it - is it possible and if not, is it a planned feature?
Yes, I see what you mean, but I agree with @codinghorror that splitting posts is a no go. But let’s look at this more closely: I’d say raising a new topic in a reply isn’t a problem per se. The problem starts when someone replies to that new topic within the old topic. (And I don’t really blame inexperienced users for doing this, as it requires quite some discipline to realize that you need to reply-as-new-topic to one part of the post but can reply-within-topic to another part of the post.)
So if we look at a situation where someone replies to a new topic within the old topic, I think a feasible solution would be to copy to new topic.
Now, it does get a bit more complicated because only the post that introduced the new topic would have to be copied while the replies would need to be moved (also because copying should be kept to an absolute minimum).
If the reply not only replies to the off topic part of the post but also also to the on-topic part, I think we just gotta leave the “mess” as it is and intervene with a reply, asking to please continue the off topic conversation elsewhere.