I understand it is best to keep the dog away from gun noise. But I am asking about dogs that must work around gun noise, where their being there is not optional.
A waterfowl retrieving dog is one example. There are many gun dogs that work in upland hunting also.
While gunfire is less frequent for police K9, MWD, and dogs caught up in self-defense shootings, they are still exposed. It's not a matter of the frequency or volume of the gunfire. I know from personal experience that one shooting is enough to cause permanent damage.
I don't know of a practical solution to this. It seems like the sporting gun dog would be the best candidate for any kind of hearing protection that could be devised, but I'm not aware of any such device that is practical.
Even if a hearing protection device for sporting gun dogs could be devised, it's doubtful that it would make sense for other dogs to wear it constantly in case a highly improbable shooting were to occur. But working dogs are still at risk.
I am familiar with police, military and personal protection dog training and sport trials based on that type of training. The dogs are often tested for gun shyness or other undesirable reactions to gunfire by using 6mm rimfire (.22) blank revolver and .32 cartridge blank gun (most of the training and trial rules and traditions were developed in western Europe). The revolvers are usually held up high in the air and fired and don't present as much risk for hearing damage as actual gunfire from a live service caliber cartridge, a carbine, rifle or shotgun.
Patrol K9's (as opposed to detection dogs) are only deployed on suspected felony cases where the probability of gunfire is significant. MWD's are more likely to be exposed to rifle fire. Service dogs assisting disabled people as near-constant companions have a higher probability of being involved in a self-defense incident than pets that are usually kept at home. Of course, any pet has some risk whether they're caught up in a home invasion, or just out for a walk.
For home defense, and for the MWD's handler, it seems practical to use a suppressor to reduce the exposure even if it doesn't make the gun "hearing safe." The investment made in a MWD is very substantial and nobody wants a deaf sentry dog. But for the patrolman or self-defender, the suppressor doesn't seem like a practical option for carry.