It’s more the jake brake than the retarder that’s banned, and that is because it is noisy, and bothers people when truckers use them in city limits, but they are making great strides at getting more and more quite.

^^^ I was going to say that!!! ;)

honorable mention

What @SQUEEKY2 said exactly.
What is a Jake brake? A Jake brake is an addition to a Diesel engine that turns the engine from a energy producing plant into an energy absorbing plant. It does it by turning the fuel injectors off during engine retarding, and releasing the compression when the piston reaches TDC on the compression stroke.
They keep the engine temperature up because, without a Jake brake, on deceleration, no fuel is injected and the engine cools down, which is not good for the engine. A glaze builds up in the cylinder of a cool engine, sticking the piston rings in their grooves resulting in a shorter engine life between overhauls. When the air is compressed it gets hot keeping the engine hot. If the compression brake did not release the compressed air, the compressed air would force the piston back down with little braking effect. If fact, an engine equipped with a Jacobs engine brake will absorb the same amount of power that it produces under normal running conditions.
Want to make your Diesel engine last longer? DON’T LET IT IDLE AT LESS THAN 1000 RPM!

Here’s a video of one in action. He turns it on at 0:19. They are quite loud and are especially annoying to anyone living near the road.

I couldn’t even stand to drive a truck that loud much less have to listen to it.
What the public fails to realize is the Jake brake is a safety device, and brake saver that’s why manufacturers are making them quite these days, some cities have long steep grades in city limits, and using the jake on these grades is safer for everyone,and prevents the trucks wheel brakes from getting too hot and taking a chance at failing, and causing an accident.
But all the public is concerned about is ,is the noise .

@SQUEEKY2 They could, and should, be made quieter. Dumping that noise into an 8” collector is just too loud.

@LuckyGuy They do make them quieter. With longer, improved mufflers. From the vehicle manufacturer!!
But that doesn’t mean the exhaust system can’t be modified to make it louder. There is just something about a Jake brake, working hard, and the sound echoing down some mountain canyon out of a couple 5” chrome stacks…

@kritiper Like the “potato potato potato” of a Harley.