There seems to be some misinformation about the ID4's slow speed sound. It is a federal government requirement for manufactures to include this on the vehicle. But as far as the research I have done it is not illegal for the owner of the vehicle to modify this sound. Just like it is not illegal to cut off the tags on a pillow. But if a hearing impaired person came in contact with your vehicle because you eliminated the sound you may be in trouble. Personally, I think the sound is obnoxious but I rarely have the windows down so I can not hear it inside the vehicle.
People should do their own research.
For the US at least, car certifications, and safety inspections are a state by state issue. You should review your state's laws to determine if there is any liability for disabling a safety system for your own safety inspections (if required).
A the US Federal level, certainly NHSTA says no manufacturer or entity subject to NHSTA review is allowed to provide for the modification, removal, etc. of such devices. I suspect that no reputable nationwide brand auto repair shop would do it, as you mention for the liability exposure alone, regardless of exposure to NHSTA oversight or not. Your shade tree mechanic might not have such qualms depending on your local.
And certainly owners are allowed to do whatever they want, again within the limits of whatever their state law requires. If you hurt someone due to disabling it, you'd most likely be tried under state law anyhow, as I doubt the Federal government would attempt to assert any jurisdiction over a singular vehicular indicident.
Excerpt from FMVSS no 141
Federal Register :: Federal Motor Vehicle Safety Standard No. 141, Minimum Sound Requirements for Hybrid and Electric Vehicles
For folks outside the US, where they are subject to usually much more stringent annual or bi-annual inspection, you should consult your local standards there as well.