At early THATCamps, t-shirts and shorts were the norm. I will say that I’ve been to other THATCamps where people were dressed more formally, in what I think of as “Education Business” (cardigans, comfy but stylish shoes, etc.). That’s especially the case at THATCamps that happen in conjunction with another event. So, these days: it depends! But I’d err on the side of casual if I were you.