You don't even need that length prefix if you use something like SDNV. Ironically this would have introduced a case where you have something like a null terminator that can overflow.

It would have also been a fair bit of overhead on those old single-Mhz machines. Still, almost everything you do with strings is slow so it probably would have been manageable. As an added bonus, it is zero space overhead for short strings (up to 127 characters) which are the majority of strings.