You could get android 11 if you rooted the phone but that will void any warranty and they won't repair it if you mess it up you can lookup on YouTube or Google how to root and install Android 11 but it's honestly not worth it at all...i would just stick to whatever the latest update you can get from your carrier is...i miss my s8 the fingerprint reader was so good lol but it'll run way faster if you just keep it on the android OS that is current from your carrier... what exactly are you trying to gain by upgrading the android edition?
I have an ATT s8 and the last official version was Android 9.
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I would keep it on android 9 even if you root the phone and can find the right os to install it would more than likely run horribly because the new android os is optimized and designed for the phones they release it to the reason is because eventually the hardware in the phone isn't capable of running the newer OS correctly you would lose more features than you would gain by trying to force an update for example i believe the s8 still had the heart rate reader built into the back and a different fingerprint sensor the new OS would not support these since it's only designed for the phones they specifically release to which don't have those features and that's just s couple examples of why it's not a good idea.... you can go compare the benchmark scores of the s8 and say the s20 and you'll see a big difference in their scores meaning trying to run the OS that's on the s20 would probably crash your phone