What to say to people who ask about my old self harm scars?

So many people are nosy dickheads.

Nobody has the right to know the story behind any scar on a person, and asking about them is insensitive and intrusive. It's really the same as asking an amputee how they lost a limb. Most people will be curious about that, but an intelligent person with a normal ration of empathy doesn't' fucking ask because it's very likely the person doesn't need to be reminded that their differentness is noticeable, they have the right not to be defined by that disability, and it's likely the last thing they enjoy is being asked to tell the story yet again and have to relive the event.

Depending on your mood and how the question is asked, I think "None of your fucking business" would be a perfectly reasonable response.

If that's a bit harsh, maybe go for "I went through a rough time a while back." Don't be reluctant to shut down the conversation or walk away if the asshat asks you for more details.