These types of lithium batteries have been used for years on computer motherboards. I've worked in IT for about a decade, taken apart lots of different systems (including some very old ones) and don't recall ever seeing a motherboard which had battery corrosion. Usually the only way we can tell if a lithium button battery is dead is to check the voltage on a multimeter. On that basis I'm inclined to say it is probably low risk. On a PC, these often have to be replaced after 6 to 8 years, but I have no way of knowing if the power draw on Stern pinball games is similar to what you see in PCs.