Industrial level input protection
From the image below taken from the Siemens S7-200 Manual you can see that there is some protection, they do not fully define the items. But it looks like the opto is there, and resistors for surge suppression and a diode for reverse polarity.



Digikey also has a nice article here.

How much protection you need is really up to you and your particular situation. Including but not limited to Cost / Board Space / Time / etc. For example maybe you won't need to have reverse polarity protection because of the way that the system is installed / used.