This is true, so far as it goes, it also violates your user agreement. You cannot legally dual boot the same Windows 7 or 8 that you used to upgrade to Windows 10. You have to buy a separate license for Windows 10 if you want to keep your existing Win7 or Win8 for dual boot and be legal about it.
It is definitely possible to dual boot any of those three versions of Windows in any combination or even tri-boot, but the license for each should be valid and legal if one is doing so.
I could have sworn this was discussed in these environs, and at great length, quite recently, though I think it was in the actual Win10 support part of the forum.
Edited by britechguy, 09 May 2016 - 12:15 PM.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
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