I think I've found a great solution which makes it as good as the one I had in Word 2003 etc. Back then you had to create a macro. I called it 'pasteunformatted' and assign a shortcut key (I used Ctrl+T).
If you go
Customize ribbon > Check 'developer' box to open up that tab, OK. Then select it.
Grab your some text to be copy/pasted from anywhere. This is just to set up the macro.
Click 'Record Macro'. Choose a name. Assign your shortcut key.
Now go Ctrl+V to paste. Then press Ctrl (release) then 't' as described by some users above. Now stop record. You now have a macro that will paste unformatted text as you may have had previously, with the shortcut key you assigned to it.
Hope that helps.