It is necessary to divide the review into several parts.

1. Suppressor can reduce the sound level of the expansion of powder gases. The suppressor is not able to reduce the sound level of the shutter mechanism and the flight of a bullet.
2. The sound level of the shutter mechanism depends on the design of the shutter mechanism and its size. In the pistol, the sound of the shutter mechanism is quieter than in the automatic pistol or in a rifle.Therefore, the effective distance of sound masking for different types of weapons will be different.
3. It should also be borne in mind that there is a level and sound propagation of the shutter mechanism and its distribution in open space. In a closed space, the sound of the shutter mechanism will be heard in different ways. The type of closed space in which the sound source is located will influence the amplification or absorption and propagation of sound beyond the confined space. For example, if the shooter makes a shot while inside a large iron hangar, the hangar will create an echo, amplify the sound and do not prevent the sound from spreading outside the hangar. Or if the shooter makes a shot while inside the small log house, the log house will absorb some of the sound and prevent it from spreading outside the house.
4. Background sound and environment. Background sound, such as wind noise, rain noise, leaves rustling, and other sources; multiple sound sources also mask the sound of a shot. Environment in the form of raindrops or thick fog, can prevent the spread of sound. All these circumstances affect and change the effect of sound propagation, which affects the distance to detect the location of the sound source.
5. By this, I would ask the question differently. Does the game have a mechanic that dynamically changes the noise level of different sound sources in the environment?

@Lex
Seeing the points you've brought up, and also considering that they have committed to making smaller details matter (e.g. when they said brighter/whiter clothes will be more visually obvious to zombies), it is entirely possible that the devs have added such attention to detail. In earlier builds (early access) pistol suppressors would entirely mask the gunshots from zombie detection, as if they'd not existed, and rifle suppressors acted pretty much the same as current builds.

It'd be great if we could get a dev in here to clarify if they added finer details such as these, but I've definitely made noises louder than a suppressed rifle that didn't attract any aggro.

@Geez Hi. At the moment, after update 1.5, a single shot from any pistol with a suppressor causes attention and a zombie attack at a great distance. Not sure, but now the detection radius for zombies is up to 80-100 meters, or more. Previously, a single shot from a pistol with a suppressor could activate zombies at a short distance, up to 25-35 meters. I can’t say what caused the activation of zombies, or the sound for them also sounds like a shot without a suppressor (the problem is described in this ticket T143724), or the detection distance from such a sound is increased.

P.S.
There is no such problem from a shot from rifles with a silencer, a single shot at a long distance does not cause detection for zombies, but makes them inspect close surroundings, this seems normal.

I also noticed that the pistol suppressor has no effect on zombies. Interestingly though, my friend has fired near me a few times, and I haven’t heard it. So it appears they are working for other players.