The earliest confirmed safety catches appear on 16th century French wheel locks. In contrast the numerous weapons diagrams of the genius Leonardo da vinci who died in the early 16th century never included safeties, and he drew a lot of guns.

Before the whee lock all prior forms of firearm relied on some kind of burning match or cord, or an actual burning poker to fire themselves, and thus naturally had a built in safety mechanism. Don't lite the cord on fire and you can store and carry the weapon loaded with reasonable safety. The main concern was making the gun fire at all. A good wheel lock was actually very reliable given good powder, but so expensive various forms of evolving 'firelocks' supplanted it at an early point, such as the snaphance. These had a safety function by which they could be stowed at a half cocked position. The snaphance is the first firelock that had the moving cover plate for the powder pan, and appeared around 1550.

Very few documents survive which explain anything about the use and employment of firearms this early, I'm not aware of any that explain safety but I'm more expert about the hardware then that sort of document. However it is safe to assume that the idea of muzzle discipline is much older then anyone implementing a mechanical safety.

It is also worthy of note that early wheel locks tended to be in the form of giant pistols, really more like a modern idea of a carbine, used by cavalry troops. Since those tended to be the rich and noble, they may have had greater concerns about safety.

And worth considering that weapons like crossbows and even swords have non trivial safety considerations themselves long before firearms became commonplace. This is really not an idea I think you can date to firearms specifically. I believe some crossbows have provisions for a half cocked loading position. But this may have had more to do with limiting stress on the bowstring when awaiting the command to fire then safety concerns.