Do you mean a data structure they only use once? Or one that's never been done elsewhere? If they only use it once, that seems like the worst effort/pay-off ratio you can get writing it yourself. And I don't think there's that many fundamental data structures out there... and even then, why would it be good to be forced to make your bespoke structure out of only arrays, when things like maps exist?

Sure, but it is also very common for C programs to contain data structures that have one use in the program, and could still be instances of a generic type. You mentioned red black trees, which are a perfect example of that.