Well for this I certainly don't know whether it's a bug or feature but on OneUI 3.1 you can tell your Google assistant to set the brightness to 0%. After that simply lock your phone and once you unlock it again it will totally be in super ultra screen power saving mode 😄😄. I mean it's like the entire screen pixels are turned off. Just make sure your "Hey google" voice is set up for this and you should probably have a device with a fingerprint scanner for easy unlock as you won't be able to tell the pin or pattern unless in complete darkness. Once unlocked again you can tell you Google assistant to set the brightness to 50%... Strange....
