…for how long has it been considered important for the people operating the guns, and is there some kind of timeline for the evolution of various forms of hearing protection?
My grandfather was an artillery officer in WWII, and he never wore hearing protection. He’s also almost completely deaf today (still alive at 95 years old!). His hearing has been deteriorating for decades. Reportedly, he usually stood to one particular side of the gun, and the ear that was closer to the gun was noticeably worse than the other.
FWIW, when I joined the U.S. Navy 25 years ago in the early '90s, hearing protection was strongly encouraged in noisy environments, like engine rooms. Not everyone wore it, though.
Uncle, artillery, sergeant, WWII. Otherwise the same. He said if he faced the gun with his head down a little - pointing the crown of his head at the gun - the helmet would deflect the shock wave around the ears and make it “not so bad”. He too was almost completely deaf in old age.
Some photos in Vietnam show artillerymen with the little plastic container for the earplugs attached to themselves somewhere.