Until the patch https://gerrit.libreoffice.org/c/core/+/173122 it was possible to press Shift+Ctrl+F6. It was removed since one niche use case should not occupy a previous shortcut in particular not hard-coded. The idea is to respond to arrow keys when it follows a .uno:SplitWindow function and adjust the splitter accordingly.
Seems necessary for keyboard use and a11y. +1