the windows subsystem is limited to the latest version of windows 10.
actually - scratch that: its limited to the now coming version of windows 10. there are (to my knowledge) no plans to ever port it to any other system. this was stated in one of the earliest interviews about it iirc
First stable release of WSL was the Windows 10 Anniversary Update, which has been both the Current Branch and Current Branch for Business, since back in November. (The Creators Update out soon includes updates/enhancements/fixes to it.)
It is not expected to be back-ported to Windows 7. (Why would it be?)
actually - scratch that: its limited to the now coming version of windows 10. there are (to my knowledge) no plans to ever port it to any other system. this was stated in one of the earliest interviews about it iirc