One has multiple possible modes for failure.

Sheer is one, but rare. I believe tensile strength is a measurement of stretch, which you might get from overtightening bolts.

On bicycles, it may be more common to break some of the harder axles than breaking the softer ones.

There is also bending which I think is captured with Yield Strength.

Since every MTB axle with a Freewheel that I've ever seen is bent, I've just assumed they are installed pre-bent to save people the trouble.

Standard coaster-brake hubs, however, likely have a lot less axle sticking out, so less leverage for bending them.

Thus, one might not have to worry about axle material as much. Still, no reason not to get tough axle material. The cones should still be harder than the axle.

From that McMaster link, 6 foot is only about twice as much as 1 foot, and could yield a bunch of axles (other than issues with thread differences).

12.9 grade, 10mm was a little more expensive, and unfortunately didn't seem to come in 1mm pitch.

I wonder if I could make a testing apparatus to test the bending strength of standard axles vs that McMaster threaded rod.