It's a barbaric practice that's still around, unfortunatley.
Crosby, pinfiring isn't and wasn't just done for bows. It was and is more commonly done for bucked shins. If the pretty pattern of dots is on the front of the leg, mid cannon, it's likely that's what it was done for, not a bow.
Pin firing for splints is stll around, too - it will be pretty clear from the location if that was the reason.
Pin firing marks for bucked shins do not concern me in terms of long term soundness; bucked shins are not truly an unsoundness, they are purely a result of a young horse worked too hard too soon while the bone is still flexible.
Pin firing for splints and bucked shins "works" because the time off given for the pinfiring to heal is the time needed for the splint or bucked shin to resolve, so the practice persists.
It does nothing to damage the nerves, the person who told you that was misinformed or perhaps confusion pinfiring with nerving.
Enjoy your new horse and good luck!