Is soundstage width a myth?

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. 

All things are possible with the$$ to spend on Quality throughout your system including cables , at bare minimum $20k asa bare minimum starting point 

$40k or up it starts to get much more immersive .  Most of us have  to save 

To upgrade each component section.

Sound "appears" to come from far left and right, well outside the speaker placement width.

It also appears behind the speakers, in front of the speakers, apparent height is also heard, and if you have a nice setup you can actually hear sound that appears to come from behind the listener.

You don't need expensive equipment to hear this.  A definitely not audiophile, fully studio manufactured, recording I use for a lot of demos is Led Zeppelin II, side one.  If you can't hear the swirling up and behind your head on track 1 your speakers/room are not set up properly.

Chesky demonstration disks can show all of this.

Width is a fundamental part of soundstage and certainly not a myth. This is basic and if you’re not at this level, something’s wrong. Just my two cents.