Not backwards compatible mostly this has seperated the hardware drivers from the os itself but unless the device system partition is repatitioned and the drive

Not backwards compatible mostly this has seperated the hardware drivers from the os itself but unless the device system partition is repatitioned and the drivers been provided by the hardware manufacturer old phones won't get updated any time soon. Mostly the newer phones that are released with Android O will have the ability to update to the latest version of Android missing a few things that your phone doesn't support like iOS does with older iPhones even if the Oem no longer procides updates..