Have an Late-2013 15” MacBook Pro - has both integrated and dedicated graphics. Flickering screen only visible on LCD (not external monitor), and the flickering was worse with integrated. The logic board was replaced about two years ago, so I hoped it wasn’t that. Decided to try just replacing the LVDS (LCD) cable - took about an hours and was suprised it worked. Well sort of. The flickering seems to be totally gone from the discrete graphics card, but switching to integrated it still flickers a little (but way less).
Using gfxcardstatus to force discrete, since I rarely use just battery power, and it’s totally usable again (for a while).