The new Mac Pro from late 2013 supports only Windows 8 and above installation via Boot Camp. Apple has updated the Boot Camp requirements on its support webpage and the latest generation Mac Pro is the first to drop Windows 7 support.
This means if you want to install Windows 7 you won’t have such an option. You’ll have Windows 8, 8.1 or later version of the Microsoft’s OS.
Apple will probably spread this change over its upcoming hardware updates on its Macs. The current generation of iMac, MacBook Pro, MacBook Air and Mac mini support the 64-bit versions of Windows 7 and Windows 8.
Maybe a workaround will make its way to those in need, and hopefully it won’t be that hard to use.
Source | Via