The deprecation has been set a few versions back. I'm still waiting on a reply for a similar question
The vertical split pane supports a use case which is rather difficult to accomplish with other components: On the left side a treeview, on the right side details about the selected item. The details may not scroll when the treeview is scrolled.
I have been experimenting with layouts and other more recent developments but ended up with scrollbars at the wrong places and a messy overall layout of the page. So I put the vertical split pane back in for now.
I really hope that a workable alternative is released before really killing the split panes.
The next major version will be Mendix 7.
Both split panes will be removed in Mendix 7. Logical alternatives are either scroll containers in your layout or layout grids in the pages themselves. Though layout grids are not re-sizable, they compensate by being responsive!