Preview at WWDC likely in June, followed by September launch.
Apple Drops Windows 7 Boot Camp Support in New Mac Pro
Boot Camp Assistant on the Mac Pro also specifies that it only includes support for Windows 8 or later, as evidenced in a screenshot from Twocanoes Software (via MacWindows) indicating that users are not able to install earlier versions of Windows. The 2013 Mac Pro is the first Mac that does not include support for Windows 7 with Boot Camp 5.
It is unclear why Apple has chosen to drop support for Windows 7 on the Mac Pro, but it could be a sign that the company intends to discontinue support for the operating system in future Macs given its advanced age.
This decision may not sit well with users, as Windows 8, released in 2012, has not been particularly popular. As of this month, Windows 8 and 8.1 only represented 10.68 percent of total worldwide OS market share, while Windows 7 represented 47.31 percent. Combined, Windows is installed on 90.84 percent of the world's computers.