For MacOS Sierra 10.12.6:
First, copy Boot Camp Assistant to your Desktop. Then using a text editor, such as XCode or Sublime Text 3, open the Boot Camp Assistant app which we copied to the Desktop, and find the info.plist file at
/Users/YOUR_NAME/Desktop/Boot Camp Assistant.app/Contents/Info.plist
Find the key "SupporedNonWin10Models", and delete the string containing your mac's model number (which can be found in "about this mac" > system report > hardware > model identifier)." So for me, I deleted
Save the Info.plist file, and resign the code in terminal using
sudo codesign -fs - /Users/YOUR_NAME/Desktop/Boot\ Camp\ Assistant.app
Now open the Boot Camp Assistant copy on the Desktop and it will give the option to make a Windows 7 or newer bootable usb instead of a windows 7 or 8 USB.