I think I've found the reason for the Diode Threshold myth:Because the diode's forward V/I curve is essentially Exponential, it is difficult to deal with using simple maths.

A student trying to calculate an exponential bias current will run into trouble using Ohm's Law.

One way of handling this is to break the curve into a number of separate straight-line segments. This is known as piece-wise approximation.

Each segment consists of a resistor in series with a voltage source. With many of these segments in parallel you can get a good approximation of an Exponential curve.

As you move up the curve, each succeeding voltage source cuts in and causes the graph to bend.

For a simple calculation, just one segment is often sufficient. This would be one resistor in series with a voltage source.

Different diode types need a different threshold voltage to handle the different curve shape.

So students are taught to calculate the diode current by putting a voltage source in series with the existing circuit resistance.

Unfortunately they then take the single voltage offset as gospel. I know that I did, way back when...

for graph example see...

https://en.wikipedia.org/wiki/File:PWL_diode_characteristic.PNG

........... Zim