Also Unix. Syscalls return null-terminated strings all over the place. So any language running on Unix has to deal with null-terminated strings.

I know because I run a userland that uses length-prefixed strings as far as possible: https://github.com/akkartik/mu

Not just the standard library, but the core language. They are what you get if you write a string literal in your code.

True. I was skipping over that, but the fact that functions manipulating null terminated strings are part of the standard library is certainly a reason in itself to consider them idiomatic.