illegal where? that's not the case anywhere in the US that I'm aware. Most any legislation about exists is limiting modification to environmental parts like catalytic converters.
Here is a handy link of all 50 state's noise regulations for motorcycles. Not every state has one, but many of the larger ones do.
Also, many modified motorcycle exhaust systems will not have a catalytic converter in order to save on weight, this is especially true for sport bikes.
Also, many modified motorcycle exhaust systems will not have a catalytic converter in order to save on weight, this is especially true for sport bikes.
https://drivinglaws.aaa.com/tag/motorcycle-noise-limits/