I did not find this as a previous request, apologies if it is.
At our hosted site (SWI-Prolog) we have begun to encourage long topics to be split but ask the users to help decide where and what to split. Also, AFAIK, only trust level 4 (admins) can split a topic.
Currently there is no formal system, AFAIK, where a user can request that a set of post in a topic be split into a new topic. As such, it seems each user invents a new way to do it each time (only several so far), some use post, some use private message, some use flags, some add as part of other message, some want me to guess what and where, some don’t even know it can be done, some know it can be done but are afraid to ask, etc.
Also while the splitting of topics is meant to be easy if all users correctly click appropriate reply, they don’t and so splitting is never as easy as it should be; I have to eyeball almost all of the post after the starting split post.
What is desired/suggested is a formal system where a user can start the split process then the final action is for a reviewer (admin, moderator or a privilege that can be grated) to either approve, decline, or modify the split. The request should include a title for the new topic and the selected post. Then when the reviewer goes to review the request, the topic should show up with all of the selected post selected as is currently done and the accept dialog stating the title.