I'll repeat it: Office 2011 (update 14.5.6) does support full screen mode in El Capitan, I have it and I use it. The green button contains two small arrows pointing outside, and when you click it, the window goes into full screen mode, menubar disappearing and all the other trappings. The only (very minor) difference with other apps is that to return to normal mode you have to make the menubar to appear by taking the cursor to the top, and click on a small blue square in the extreme right of the menubar (other "regular" apps return to normal by clicking the same green button again, now with the two small arrows pointing inside, once it appears by moving the cursor to the top).
However, the split mode for Office 2011 simply does not exist. You click and hold the green button, and nothing happens, until you un-click, at which point it goes into full screen mode. The ghost rectangle and mini-mission-control by which other apps go into split screen mode do not appear in any of the Office 2011 apps.
To summarise: Office 2011 - in its latest update - does support full screen in El Capitan. Why, then, it does not support the very useful split screen, is a mystery to me, because it seems that all necessary elements are there. Besides, MS Onenote works exactly like the native apps. Somebody pointed out that it is not part of Office 2011, which surely it is true, but certainly is a Microsoft product and has a clear family ressemblance. Will Microsoft implement split mode in Office 2011?