I'm not exactly standing behind it - just saying what I've read. I'm confident nuclear plants are after 9/11. Anything else is probably hit or miss, including petro/gas pipelines, coal, and generating plants specifically. Plus if a bad actor (likely state sanctioned) decides to, they can get through air gaps with spies/traitors/unwitting idiots with a simple USB drive. After air gapped uranium processing centrifuges were wrecked with an errant USB drive, I would expect all systems to disable or remove USB drive connectivity, but I'm sure that's inconsistent... at best.