Is soundstage width a myth?

EBM excellent answer.

     Whenever imaging or soundstage are mentioned, I like to remind people about these resources: The following provide tests, with which one may determine whether their system actually images, or reproduces a soundstage, as recorded. 

      ie: On the Chesky sampler/test CD; David explains in detail, his position on the stage and distance from the mics, as he strikes a tambourine(Depth Test).

     LEDR test tells what to expect, if your system performs well, before each segment. 

    Chesky CD contains a number of tests, in addition to the LEDR.

 (https://www.audiocheck.net/audiotests_ledr.php) 

 and (https://www.amazon.com/Chesky-Records-Sampler-Audiophile-Compact/dp/B000003GF3)

     The shape of your ears’ pinnae is also a variable, regarding your ability to perceive images/locate sounds.

     A Stereophile article, that explains the LEDR test: http://www.stereophile.com/features/772/ 

Yes, this Chesky CD is the one I referred to.  Most music CDs cannot be so simply recorded and would not produce such specifics cues for depth.  Mostly, good recordings don’t convey specific information about depth position of the person or instrument, but they do give one a sense of that person or instrument playing in its own space and not just pasted into the same flat plane.  This separation and the sense that some instruments are at or in front of the plane of the speakers and some are at or behind the plane of the back wall is good depth presentation.

no. neither is depth or height. 

Only a laymen’s experience to share. The experience is one of psychoacoustics. Creating enough of an illusion that make it easier for you to imagine depth. Width on the other hand is a function mostly of the recording engineer when balancing between left and right output for a given signal/instrument.

For an amazing image width illusion (no idea how/why it works) listen to Roger Waters album, Amused to Death. Just the first 2 or 3 tracks. You’ll hear a dog barking far to the right of your right speaker. Almost as if it’s coming directly to your right side. You’ll hear spoken dialogue (track 2 as I recall) that comes from beside you to the left. I use that album to set up my speakers. The illusion is almost spooky when dialed in.