What type of z transform is this?
Have a look at these notes.

In particular, this slide:



shows that using a backward difference model sets up the approximation:
$$
\frac{d}{dt} \approx \frac{z - 1}{Tz} = \frac{1 - z^{-1}}{T}
$$

There are many ways to approximate the continuous-time derivative with discrete-time difference operations. That's all that substitution is doing: choosing one way of making the approximation.