Actually, was just searching for something related, and this is apparently not true. They usually won't want to, but Exarchs can Bonesing if they think they need to.
Sometime probably not too long after the Fall, according to his background book Maugan Ra kidnapped the greatest living/surviving Bonesinger, an Eldar called, Kaeleth-Tul, from Craftworld Kinshara, and forced him to teach the Phoenix Lord Bonesinging for a few decades, until Maugan Ra knew enough to create his signature weapon, the Maugetar. He then thoroughly mutilated Kaeleth-Tul to make sure that the Bonesinger would never be able to create a greater weapon.
He's clearly a true follower of the Path of Khaine. And someone we should be wary of.
Given that many of the other Craftworld Eldar manage to stick to the paths how alien and unnatural the Paths are to their nature, we and they can probably tolerate some quite annoying social nudges to prod them to innovate.
I'd agree that any Bonesinger that goes onto the battlefield should probably be buttoned up in a very safe tank or wearing Ilthilmar, or down the line, Ithilmar plus once we've miniaturised Grav Fields and have a power armour version of Conversion fields.
