Lots - gov.uk writes for 9 year-olds (by which they mean people with a vocabulary of 15k words). Even in technical fields, there's research to suggest people respond better to plain English. Try not to make your users think too much about what you mean. Plain English also goes for plain communications, which means picking the right medium; don't write an essay length guide for 14 year-olds who like TikTok; don't create a TikTok dance to explain nuclear fission - or maybe do!