Sensing 12v line with 5v MCU

[floobydust]

Typical levels are 12V vehicles +100V and -150V spikes max., 24V trucks are +200V and -600V spikes max.

I understand the 600v PIV in your design to be super safe on the spikes, but why the 1A rating?

The 600V 1A S1J diode is a jellybean part, lowest cost. Smaller diodes are to 400V max. and not much for popular parts <500mA.

Using the PC817 opto-coupler will work too. You have to design for CTR dropping due to age (-50%) and at high temperatures.