I haven't quite found the definitive answer to this particular query; so...
take a hypothetical situation and imagine that you fit some aftermarket exhausts - to a Bonneville for example - that have 'not for road use' etched on them; but sound so much better.
Are they, strictly speaking, 'illegal'
Should the eventuality arise, will having them on the bike, be bad news if you should need to make a claim.
Will the insurance company be able to wriggle out of paying a claim, unrelated to the exhausts.
Worst case, are effectively uninsured with them on.
Anyone have the answer?