At what point would one consider an opto or magnetic isolator for driving an ULN2308 input? It is all on one piece of silicon after all and most stepper motor voltages will fry the controlling logic dending on the failure. Same for other higher power dedicated single chip drivers. This isn't something I recall seeing in prop or many other discussions. For example, power drive circuits where an FET punches through and takes out everything back to the opto isolators output including pcb traces in a particular medium frequency high voltage generator board.
I ask this tangent to the thread regarding stepper drive and yet another dead prop to see what others here think, do or perhaps, do not. And also circuit protection (and human) is a topic that does not seem to come up much on many forums such as this one. That was a lower power application, but some of the other projects in the forums can be at significantly higher power (AC line, high currents from battery banks etc.)