Places and situations where you can't accommodate variable-length encodings. As far as future-proofing, UTF-8 is essentially the new ASCII, in that UTF-8 will remain a backward-compatibility goal for any other format that will succeed it.
> As far as future-proofing, UTF-8 is essentially the new ASCII, in that UTF-8 will remain a backward-compatibility goal for any other format that will succeed it.
Yes, I love that every byte transmitted on the Internet still reserves code points for controlling teletype (or similar) machines.