And, again, in practical terms, once you make the "angle" in the
(near) square wave you have cut/pressed sharp enough, a mechanical
stylus just won't track it. The angle of the groove's motion
side-to-side, the V-shape of the groove, and the shape of the stylus
(typically an elliptical cone) will be such that most of the
mechanical force generated by the pressure of the stylus on the groove
will be upwards rather than sideways. The stylus tip will accelerate
upwards more rapidly than it will accelerate sideways, and it'll pop
out of the groove entirely.

The Sheffield Direct to Disc pressing of the 1812 Overture was
notorious for this... the "fire the cannon" passage would pop almost
any cartridge out of the groove. It wasn't quite at the point where
it was literally impossible to track... but it was a *really* severe
test of a cartridge's ability to follow a groove.

I think you're thinking of the Telarc disk. (I don't remember a Sheffield
"1812".) I don't know what the waveform "looked like", but I doubt it
approximated a square wave.

I do remember it knocking the pickup out of the groove. Not only did you
need a good-tracking pickup, but the arm-mass / stylus-compliance resonant
had to be very low -- so low that it would not normally be considered a
reasonable resonant point.