Some distributions were using patched versions before, but from 17 Apr 2014 - screen 4.2.0 and further have it.
Shortcut is: C - a |
I usually use screen in a very vanilla way (i.e. logged into some rhel box I didn't build or configure) and in my experience you still cannot split vertically. x_x
ctrl-a | is there from long more than 3 years, in the manual page for those willing to read it.
Unfortunately for me most of the applications I have access to (and want to have installed) do not have custom patches including functionality like that. I'm left with C-a S as my only native option.