Version of screen in raspbian has this functionality. I thought that Arch being rolling release it would have all the latest software, so why doesn't it have this?
The vertical split in Screen isn't a part of its default functionality. It's a user patch that several distros keep in their repository to go with Screen. Since Arch usually keeps a more update, but 'pure' repository of software, you're missing the patch.
I don't think arch keeps the patch in their repositories, but from some Googling, you can download it here.
In my opinion, screen pails in comparison to Tmux. It has a lot of the features you want built in and is far easier to use. It might be worth a try ;)
Edit: To install the patch, install it like you would a standard Linux application form source.
- Run "./configure"
- Run "make"
- Run "sudo make install"
- Delete the folder you extracted the patch to and its archive as they're no longer needed.
Edit2: According to goldilocks (see comments), this will patch the applications binaries, and is not a plugin for an existing installation of screen. This option will require downloading the source for screen and compiling the two together. If you need help with that, please create a second question.