Interestingly, this also happened to a lot of Vaios when people upgraded to Windows 10. Now, your Linux was eventually fixed while my Vaio is still running at 0% backlight with zero response from Sony.
I think I know what happened in windows: windows itself has no backlight driver, and the standard GPU drivers have no backlight handling (or do, but its disabled by default). So previously, these laptops would ship with a special Windows 7 driver that would handle the backlight. But its "so much work" to port that driver to windows 10... so they don't. So in this scenario, my laptop originally worked. A fresh installation would require proprietary Samsung drivers, and samsung has stopped providing those for windows 10... literally no fix available (it's possible the windows 7 drivers still work? to be clear, Ive never checked if there are work arounds or other drivers available. I switched to linux before windows 10 came out, and my dad is happy with linux resolving his backlight issues, so Ive never had to look it up)
I moved my laptop to Linux full time around the Windows 8.1 transition (which had also re-broken the drivers somehow). Everything worked great in Linux for me until kernel 4.2 or something when backlight brightness stopped working. Then after a few weeks or searching, I found a parameter that made it work again.
After applying the fixes to my dad's laptop, and setting up a few of his "must have" programs under wine for him, he claims that Ubuntu runs better for him than Windows ever did.