iOS backup apparently doesn't work like you described. E.g., when restoring a backup from iOS 14, it doesn't revert your system to iOS 14. Clearly there's a separation of restoring the executables and restoring the configs.

I do expect "the config/setup that would trigger a battery drain in <iOS X" to be restored, but Apple can always fix iOS so that the same config/setup won't break >=iOS X.