My response was specifically targeted towards the discussion of this thread (iPhone X) rather than expanding my hypothesis to other devices but you raise a good point mate.
IMO, devices using curved edges are misleading and confusing when they adopt the diagonal measurement.
oh.. those pictures of other phones i posted weren't supposed to be about the curved corners/edges.. the point was that they're not 16:9 aspect ratio.. they are longer/taller (or widescreen if viewed in landscape)
for example, if we follow the suggested logic-- find the unobscured 16:9 ratio within the screen then give the diagonal measurement of that area... with the S8, you're saying we should measure the screen according to the red outlined area:

..there's a lot of screen being left out of your calculation since the screen's aspect ratio is wider than 16:9..
doing the same thing with the X:

..that's the 5" measurement you're talking about.. that's the 5" diagonal of a 16:9 rectangle on an iPhone X..
if you take out the notches/corners of an X, you're left with a 2:1 ratio with a 5.5" diagonal:

but even if we were to say "find the largest unobstructed rectangle on a screen, regardless of aspect ratio, then give the spec of that"... that's still an inaccurate measurement.. there's still more screen that's being left out of the calculation so it's false.
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or, at least an understanding of why in the past, a diagonal measurement was even used in the first place..There needs to be some way of physically comparing against past technology
first, here's why a diagonal tells almost nothing:

the square on the left is 12 x 12 with a 17" diagonal... it's area measures 144 square inches (a square foot)
the rectangle on the right also has a 17" diagonal except its area is only 68 square inches..
they both have the same diagonal except the square is more than 2x larger.. the diagonal measurement tells us nothing of how big/small a rectangular shape is.
what the diagonal measurement does tell us requires going into the past.. when TVs had tubes.. the tubes were circular and then a TV frame/bezel was added around that in order to give us a rectilinear view.. in essence, we used a rectangle to crop a circle.. so, when you do that with the above examples:

..you see both shapes are encompassed by the same size circle.. both shapes, if it were an old style TV, would require the same 17" tube.. that's what the diagonal measurement means.. that's when the diagonal measure makes sense to use since it's describing an actual piece of hardware.. the tube.
but with LCD etc? it makes no sense to use diagonal measurements since they tell us almost nothing about the hardware or screen size.
What is your suggestion to compare those devices you listed, including the iPhone X?
use AREA.. i mean, that's how we measure anything else..
if you size up a house, you're going to arrive at a square footage...
so the house is say, 1500 square feet.. then you decide if you like the layout or not.
two houses could be identical in size but very different in floor plan/layout.. pick the one you like better but realize they're still the same size..
do the same thing with phones.. describe the screen size as it's area.. then pick the layout you like better.
in the case of iPhone X vs 8...
the screens are the same size.. pick the shape you like better.
but yeah-- use area instead of diagonal.. that is my suggestion.
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