This is one of the few options I am missing from SMF. I know the description sounds weird but it is exactly what it is called in SMF.

For large topics having several pages, you sometimes need to split it somewhere from the middle. With this option you could split it in a sescond and make a new topic from the remaining messages . I have looked into it in both 3.4.7 and 4.0 but I could not find a similar option and I thought it could be fine to have

Yes - this is something I've been missing too. It was an option we had in phpBB - and it is certainly missed.