Android - How do I simulate menu button on Android 6?
As per this post, go to Settings → Accessibility → Dexterity and Interaction → 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.
I was able to get the long press of the back button on my Galaxy S7 to open the menu on a really old app by forcing the keyboard to open first. My app "eWallet GO!" has a Search field that I was able to click in to open the keyboard. After the keyboard (Swype in my case) opened, I held down on the back button and the app's menu opened. Previously, the whole app had closed when I tried holding down the back button.
Hope this helps.
For the Galaxy S6 (and Galaxy S7 too), you don't need to open a keyboard. Touch and keep touched the back button for at least 2 seconds and the menu appears.
This also works on a brand new Samsung Galaxy J7 running Android 7 (November 2017).