> Anyone have any ideas about which open source codebases UTF-8 validators exist in?

Rust's std library -- the canonical way to read a text file to a string is to (implicitly) use `std::str::from_utf8`. If I remember correctly, the current implementation doesn't use SIMD specificially but will of course contain vectored instructions if the compiler can select them on the platform you target.

I did a comparison with another SIMD based implementation last year. Maybe it's time to update it: https://github.com/killercup/simd-utf8-check