What is the oldest Posix script that I can still run in a modern shell?

The question is not about the oldest. I wrote Bourne shell scripts in the mid-80s which would probably still run today under BASH. But they were pretty trivial, and only used basic UNIX commands.

What you want is the oldest scripts that do actual meaningful work, and yet still run because the person who created them took some care in how they did things, and the people who wrote the shells took care not to unnecessarily break backwards compatibility.

And for that question, I have no answer.