Don't know what the statistics are for autistic self-harming. Apart from smoking tobacco I wouldn't dream of doing it, and I stopped smoking years ago, and even that wasn't "proper" self-harming, it was an addiction.
I just looked it up. Apparently it does seem to be more prevalent for autistic folks than for the general population, but it's the kind of behaviour that might well be under-reported, so the statistics might not be all that valid except as minimum percentages.
Apparently it's important to approach the topic with sensitivity and understanding, so I won't say much more because thinking about it creeps me out and the reasons for self-harming are manifold and complex. I don't feel any stigma towards people who do it, I just don't want to think about it too much.