How to add black borders to screens wider than 16:9

No_Border works differently, but should still fill the screen. But, you want a stage size ratio that can cope with the range you want to support. I've previously used a 14:9 stage, and that's enough content to take care of a 16:9 or a 4:3 screen without cropping into it too much.

Sideline things never get revealed with No_Border, so, to support 19.5:9 the easiest solution is to have a stage that was, for example 1950x900, and where only the middle 1200x900 was important. The other 750 pixels would need to be background that it doesn't matter if the user never sees it, so that iPad users wouldn't miss out.

Buttons that need to be on the left or right edges of the screen can be positioned using code, to make them be on the edges on all screens.

You could make your stage be 16:9, so 1600x900 for example. Again, 1200x900 of that is what iPad users would see, and iPhone X users would see 1600x738 of the stage. That could be good enough.

If you did change to StageScaleMode.SHOW_ALL, or just remove any stage scale mode code (because showAll is the default anyway), then you would want a stage that is the width and height of the important content, and also have bits off the edges that might get revealed. That's the mode I generally use, and for an action game I would have a 16:9 stage that had extra content above and below for iPad uses to see. For a book like app, where I need them to see the whole of the vertical page (in a landscape app), I have the stage at 4:3, with extra content to the left and right.

I am currently updating an app that is that way, and so I will have to add in extra content to the left and right of the 16:9 stage, to cope with S8 and iPhone X.

From the sound of it, I think your best option to try first is a 16:9 No_Border stage, and see if the crop into the left and right on iPad, and top and bottom on S8 and iPhone X, are acceptable.