The split panes were deprecated a little too aggressively in 5.18. We will bring back the possibilty to create them.
However, the current thinking is that we do not want these widgets to be part of the base widget set in the next major version. We think they do not belong in modern interfaces.
Maybe the community can come up with a way to add draggable splitter to an existing container widget.
I guess that you could replace this with the responsive classes of bootstrap. Allthough that still wouldn't make your pane resizable. However it would make it fit for multiple situations.
I don't understand the depreciation of them. If you use flexible forms that adapt to the use of data the split panes are very useful. E.g. if you want to show an image in the left part or the right part depending on an eNum it will not be possible to resize the pictures. Please reconsider depreciation of it.