It's always been turned-on.. suddenly I couldn't see it anymore... such a malfunction is unable to affect me playing this great game but the kill move was so cool to give up. I didn't touch anything on game play option since buying it. And tutorial or info messages sometimes appears although I already turned these off on HUD setting..
Perhaps disable this feature, play a bit with that setting. Then restart the game and enable it again. You can also Verify/Repair the game with GOG Galaxy (or Steam), just to make sure nothing else went wrong.
But I don't think there's that much else you can do here.
If devivre`s suggestion doesn`t work then you can make sure that the finishers are enabled through the witcher 3 ini . Navigate to your Witcher 3 installation folder or to be more precise The Witcher 3>bin>config>base and look for the gameplay file. Note that you will need to open it using Notepad. Then look for AutomaticFinishersEnabled=true . If it says false then highlight false and change it to true . Then click File at the top and click it and select Save. Then retry the game.