When a topic is split, the newly created topic goes in the same board. This mod adds a dropdown box to the split page that gives the possibility to select a different board for the new topic. When the newly created topic is "moved" to a different board, then a "new topic notification" is sent to the member with new topic notification active on the destination board. Thanks to scottfrey for the idea.
This mod adds also the option (Admin > configuration > Modification Settings > Miscellaneous) to post a message to the old topic pointing to the newly created and a note in the first message of the newly created pointing to the original topic. Thanks to Guillelmus for the idea.