When an operating system introduces new features software has to be updated to use those new features. Software cannot magically do things they were designed to do just because an operating system update was introduced. Office 2011 can be used in fullscreen mode but not split screen. All software that supports split screen has been updated to support it.
Microsoft has released Office 2016 for the Mac which fully implements fullscreen mode and split screen. The test for support is quite simple. Open a window, then click and press the green widget. If your screen displays a blue overlay on either the right or left half it supports split screen. If it does not show the overlay the program does not. Office 2011 does not. Microsoft will continue to support the 2011 version with security updates and bug fixes for another year or two but it will not get new features.