It's confusing, I agree, but I am thinking of it like being written down is necessary for surviving, but not sufficient. There may be other needed criteria for the myth to survive (like what it was written down on couldn't have been damaged, or people had to pass the written down myth from generation to generation), so just being written down is not sufficient. On the other hand, if a myth survives, that's sufficient to say it was written down because all myths that survived were written down. So the sufficient clause is surviving and the necessary clause is written down.

So I guess it's helpful to reword the statement to, "All oral myths that have survived are the ones that were eventually written down." It doesn't change the meaning of the sentence and also shows that it's a straightforward group 1 indicator. You could also say, "The oral myths that have survived are only the ones that were eventually written down" which also shows the necessary condition as "written" following the "only" phrase and doesn't change the meaning.

Would it be correct for the negation of U --> PR to be /U --> PR? #help

I like to also think of it like the test can't expect you to know everything out there about every topic. So if it brings up an argument about art history, it's not assessing whether you know if the claims about art history being made are true, but whether the argument itself is valid.