I have an application, which uses MENU button for navigation purposes. Some devices doesn't have this button. I have tried long pressing app switch button, but it is not working.
It depends on device.
Device has menu button
If device does not have menu button then Menu items appear on
App ActionBar and/or with overflow button
This used to drive me crazy. I have an app where it is critical to be able to use the button. I was sitting, and my headset cable fell across my phone, and I saw the button pop up.
After playing around I found out, that pressing the two outer buttons, and holding them pops up the menu button!
I was on a Samsung Galaxy S7 with Android Nougat and Oreo with this issue. On an S9 with 8.1 Oreo there is an extra button on the toolbar for this menu. I.e. It came back on my new phone.