Nothing. There is no coil to induce a voltage.
I think the issue is if there is an inductor in the circuit. If the optocoupler is reverse biased with a high enough voltage, it will conduct, and possibly get crispy. Here’s a nice explanation, but not referring to SSRs specifically.
Here’s a quote from:
How Freewheeling Diode Work: Easy Explanation (etechsparks.com)
"Flywheel diodes are in fact clamping diodes served to provide a DC discharge path for the remnant stored energy from an inductive load such as a motor or a solenoid. In principle, this is counteracting the back emf due to Lenz’s Law for any devices built on the principle of electromagnetism such as an inductor or a soleniod. In short, it is a protection mechanism.
Usually, a transistor or a current driver is connected at one end of such inductive load. Without flywheel or clamping diode, such remnant energy can surge the transistor, hence causing it to breakdown."
$.02 for insurance, if any inductive loads are in the circuit, is well spent imho.