12 volt happens to be easy to do with lead acid batteries (as well as other multiples of 6v so it ended up in cars. It's really too low for most serious high current apps, too high for electronics, though very safe and very common. Today, it's too low to charge newer laptop batteries directly, it needs to be boosted to do that. (Indeed, while it is the voltage of the car battery, you need more than 12v to charge that, so the alternator gives you that and the battery regulates it down.)

Anyway point is that power regulation and conversion has undergone a complete revolution so we should get rid of that legacy stuff soon. It might be time to do a serious round of tests to find out what the highest voltage is that's safe, and what the highest voltage is that's not too unpleasant, and work with those.

Now safe is an interesting question. I've heard that 60 volts is safe for most people but it can give you a nasty "ouch." 12 volts is something most people don't feel at all. I don't know if the safety threshold changes much for people with older pacemakers and implants -- I would presume modern ones are designed to handle higher voltages.

Then use the "Safe but hurts" for medium power applications. Use "Can barely feel it" for lower power applications or ones where things get wet or people are regularly touching exposed terminals. Use "This could injure you" for household heavy duty applications and "This can kill you" for industrial/transmission applications. Transform them all down the real voltage the electronics want, which is generally below 5 volts, often down to just 2 or 3.