My UN65KS8500 also just got the dark shadow that runs vertically close to down the middle of the screen. Appears to be the exact same issue others are having here. I bought the TV back in 2016 and have always been a loyal Samsung customer, but am disappointed that this expensive TV hasn't lasted. it seems to be a known issue with several of these models. I know my TV is out of warranty, but I would love if there was some way for this issue resolved. I would have expected the TV to have lasted much longer without this kind of issue, as others have stated. I have attached some photos of the issue. Thanks in advance for any assistance.
I would love to assist you with this Does this happen on all sources?
Navigate to Settings > Support > Self Diagnosis > Picture Test, and then follow the on-screen directions. Do the lines appear during the test?