Talking a bit out of my ass here as someone whose EE experience ended at graduation, but a good general rule is if you stay below 5 mA continuous, it is rather difficult to injure yourself. For extremely short shocks (like you get when you discharge a capacitor across yourself) you can get away with a lot more than that, though.

I would be rather surprised if a device that is powered off a lithium ion battery like the one in the Flipper Zero managed to seriously injure someone.