P-Strings (Pascal) do exactly this. The original ones limited it to 255 (instead of using the pointer size), but others used more. One of the nice things about using Pascal for programming back In The Day.

The article says it doesn't know where ZTSs came into Unix, but I thought I read somewhere, many moons ago, that it was a pretty simple choice: prefixed strings used one more byte of memory than a prefixed string for a string longer than 255 characters, and on the systems at the time, that mattered a lot.