How do I simulate menu button on Android 6? I'm using Samsung Galaxy S7. Long-pressing the recent button does not simulate menu button but activates split screen mode. Long-pressing the back button doesn't work either; it simply closes the app. I'm using an old app that requires the menu button. Any solution to this without rooting the phone?
As per this post, go to Settings &rarr; Accessibility &rarr; Dexterity and Interaction &rarr; turn on the Assistant Menu.  This will add a floating button to the screen that you can press to reveal a menu containing the old Menu button.

Samsung also says "In certain applications, you can touch and hold the Recent Apps button to view the Menu options."  This is probably limited to newer apps as well which is why it didn't work for you.